Is it possible to change the size of an arrowhead in TikZ/PGF?

Question

I'm trying to draw an arrow with a bigger head than normal. Is it possible without changing the style of the line itself?

Answer 1

Update 2013-02-27

This code is now superseded by the new arrows.meta library bundled with TikZ 3.0.0.

See an example of use in Ignasi's answer below.

---

Update 2013-01-23

The development version of the complete code is now available on sourceforge. The old arrowhead key is now replaced by arrow head to give consistency with the other keywords in the package.

---

Update 2012-11-13

I edited all the predefined arrow tips and those in the arrows library, except for the "almost 0d" ones (serif cm, space and all the line caps arrows) and implies (because I think it depends indeed on the line width), whose original code has been included for completeness, so that one can find all the previously available arrows in just a single code.

For "almost 1d" arrow tips (hooks like, and bracket like), the arrowhead value sets the height of the tip, instead of its length (for left and right variants, the height of the whole hook).

The code of the to new arrow has now been fixed. For 45 degrees arrows, the amplitude of the tip is actually 45° and not 46° as in the original code.

Of course, you can use the new arrows as markings too.

I can't upload the new code because it exceeds the size limit for the answers, but I'm posting a picture of the result of the new test file. I sent an email to Till Tantau asking to turn the code in a new TikZ library and we'll see.

---

I'm not a TikZ guru, so I've done what I could. Therefore everyone is invited to improve the code, and I really think it can be improved. Save the new arrows code as arrowsnew.tex and load it by means of \input{arrowsnew} in your preamble. There are nine new arrows named latexnew, latexnew reversed, latex'new, latex'new reversed, onew, squarenew, stealthnew, stealthnew reversed and tonew (or >new). It works like this: if you call an arrow without the arrowhead key or you don't assign it any value, the original code is used, so that the new code can fully replace the old one. Otherwise, if you assign a value to the arrowhead key, the lenght of the arrowhead will be set to <value>. At the moment, the tonew arrow is slightly different from the original. Please, don't directly edit the code, but feel free to reuse/edit it in a new answer.

The result of the new code 2012-11-13

The old test code

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows}
\input{arrowsnew}

\tikzset{options/.code={\tikzset{#1}}} % just to compact the code

\begin{document}
\begin{tikzpicture}
\draw [red] (2,-24.2) -- (2,0.2) node [above right, black] {1 cm};
\draw [red] (3,-24.2) -- (3,0.2);
\foreach \x [count=\i from 0] in {
  {latex-latexnew},
  {latex-latexnew, arrowhead=1cm},
  {latex-latexnew, arrowhead=1cm, line width=1pt},
  {latex reversed-latexnew reversed, arrowhead=1cm, line width=1pt},
  {latex'-latex'new},
  {latex'-latex'new, arrowhead=1cm},
  {latex'-latex'new, arrowhead=1cm, line width=2pt},
  {latex' reversed-latex'new reversed, arrowhead=1cm, line width=2pt},
  {o-onew},
  {o-onew, arrowhead=1cm},
  {o-onew, arrowhead=1cm, line width=2pt},
  {square-squarenew},
  {square-squarenew, arrowhead=1cm},
  {square-squarenew, arrowhead=1cm, ultra thick},
  {stealth-stealthnew},
  {stealth-stealthnew, arrowhead=1cm},
  {stealth-stealthnew, arrowhead=1cm, ultra thick},
  {stealth reversed-stealthnew reversed, arrowhead=1cm, ultra thick},
  {to-tonew},
  {to-tonew, arrowhead=1cm}
}
{
  \draw [options/.expand once=\x] (0,-1.2*\i) -- (3,-1.2*\i) node [right] {\x};
}
\draw [<->new, arrowhead=0.25in, line width=4pt] (0,-24) -- (3,-24)
      node [right] {\verb!<->!new, arrowhead=0.25in, line width=4pt};  
\draw [red] (3,-23.2) ++ (-0.25in,0) -- ++(0,-1.6) node [below,black] {0.25 in}; 
\end{tikzpicture}
\end{document}

The result

The old arrowsnew code (superseded)

% This is not an official TikZ library. Use at your own risk!

\makeatletter
% alternative latex arrow
\pgfarrowsdeclare{latexnew}{latexnew}
{
  \ifdim\pgfgetarrowoptions{latexnew}=-1pt%
    \pgfutil@tempdima=0.28pt%
    \pgfutil@tempdimb=\pgflinewidth%
    \ifdim\pgfinnerlinewidth>0pt%
      \pgfmathsetlength\pgfutil@tempdimb{.6\pgflinewidth-.4*\pgfinnerlinewidth}%
    \fi%
    \advance\pgfutil@tempdima by.3\pgfutil@tempdimb%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{latexnew}%
    \divide\pgfutil@tempdima by 10%
  \fi%
  \pgfarrowsleftextend{+-1\pgfutil@tempdima}%
  \pgfarrowsrightextend{+9\pgfutil@tempdima}%
}
{
  \ifdim\pgfgetarrowoptions{latexnew}=-1pt%
    \pgfutil@tempdima=0.28pt%
    \pgfutil@tempdimb=\pgflinewidth%
    \ifdim\pgfinnerlinewidth>0pt%
      \pgfmathsetlength\pgfutil@tempdimb{.6\pgflinewidth-.4*\pgfinnerlinewidth}%
    \fi%
    \advance\pgfutil@tempdima by.3\pgfutil@tempdimb%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{latexnew}%
    \divide\pgfutil@tempdima by 10%
    \pgfsetlinewidth{0bp}%
  \fi%
  \pgfpathmoveto{\pgfqpoint{9\pgfutil@tempdima}{0pt}}
  \pgfpathcurveto
  {\pgfqpoint{6.3333\pgfutil@tempdima}{.5\pgfutil@tempdima}}
  {\pgfqpoint{2\pgfutil@tempdima}{2\pgfutil@tempdima}}
  {\pgfqpoint{-1\pgfutil@tempdima}{3.75\pgfutil@tempdima}}
  \pgfpathlineto{\pgfqpoint{-1\pgfutil@tempdima}{-3.75\pgfutil@tempdima}}
  \pgfpathcurveto
  {\pgfqpoint{2\pgfutil@tempdima}{-2\pgfutil@tempdima}}
  {\pgfqpoint{6.3333\pgfutil@tempdima}{-.5\pgfutil@tempdima}}
  {\pgfqpoint{9\pgfutil@tempdima}{0pt}}
  \pgfusepathqfill
}

% alternative latex reversed arrow
\pgfarrowsdeclarereversed{latexnew reversed}{latexnew reversed}{latexnew}{latexnew}

% alternative latex' arrow
\pgfarrowsdeclare{latex'new}{latex'new}
{
  \ifdim\pgfgetarrowoptions{latex'new}=-1pt%
    \pgfutil@tempdima=0.28pt%
    \advance\pgfutil@tempdima by.3\pgflinewidth%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{latex'new}%
    \divide\pgfutil@tempdima by 10%
  \fi%
  \pgfarrowsleftextend{+-4\pgfutil@tempdima}
  \pgfarrowsrightextend{+6\pgfutil@tempdima}
}
{
  \ifdim\pgfgetarrowoptions{latex'new}=-1pt%
    \pgfutil@tempdima=0.28pt%
    \advance\pgfutil@tempdima by.3\pgflinewidth%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{latex'new}%
    \divide\pgfutil@tempdima by 10%
    \pgfsetlinewidth{0bp}%
  \fi%
  \pgfpathmoveto{\pgfqpoint{6\pgfutil@tempdima}{0\pgfutil@tempdima}}
  \pgfpathcurveto
  {\pgfqpoint{3.5\pgfutil@tempdima}{.5\pgfutil@tempdima}}
  {\pgfqpoint{-1\pgfutil@tempdima}{1.5\pgfutil@tempdima}}
  {\pgfqpoint{-4\pgfutil@tempdima}{3.75\pgfutil@tempdima}}
  \pgfpathcurveto
  {\pgfqpoint{-1.5\pgfutil@tempdima}{1\pgfutil@tempdima}}
  {\pgfqpoint{-1.5\pgfutil@tempdima}{-1\pgfutil@tempdima}}
  {\pgfqpoint{-4\pgfutil@tempdima}{-3.75\pgfutil@tempdima}}
  \pgfpathcurveto
  {\pgfqpoint{-1\pgfutil@tempdima}{-1.5\pgfutil@tempdima}}
  {\pgfqpoint{3.5\pgfutil@tempdima}{-.5\pgfutil@tempdima}}
  {\pgfqpoint{6\pgfutil@tempdima}{0\pgfutil@tempdima}}
  \pgfusepathqfill
}

% alternative latex' reversed arrow
\pgfarrowsdeclarereversed{latex'new reversed}{latex'new reversed}{latex'new}{latex'new}

% alternative o arrow
\pgfarrowsdeclare{onew}{onew}
{
  \pgfarrowsleftextend{+-.5\pgflinewidth}
  \ifdim\pgfgetarrowoptions{onew}=-1pt%
    \pgfutil@tempdima=0.4pt%
    \advance\pgfutil@tempdima by.2\pgflinewidth%
    \pgfutil@tempdimb=9\pgfutil@tempdima\advance\pgfutil@tempdimb by.5\pgflinewidth%
    \pgfarrowsrightextend{+\pgfutil@tempdimb}%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{onew}%
    \advance\pgfutil@tempdima by -0.5\pgflinewidth%
    \pgfarrowsrightextend{+\pgfutil@tempdima}%
  \fi%
}
{ 
  \ifdim\pgfgetarrowoptions{onew}=-1pt%
    \pgfutil@tempdima=0.4pt%
    \advance\pgfutil@tempdima by.2\pgflinewidth%
    \pgfutil@tempdimb=0pt%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{onew}%
    \divide\pgfutil@tempdima by 9%
    \pgfutil@tempdimb=0.5\pgflinewidth%
  \fi%
  \pgfsetdash{}{+0pt}
  \pgfpathcircle{\pgfpointadd{\pgfqpoint{4.5\pgfutil@tempdima}{0bp}}%
                             {\pgfqpoint{-\pgfutil@tempdimb}{0bp}}}%
                {4.5\pgfutil@tempdima-\pgfutil@tempdimb}%
  \pgfusepathqstroke
}

% alternative square arrow
\pgfarrowsdeclare{squarenew}{squarenew}
{
 \ifdim\pgfgetarrowoptions{squarenew}=-1pt%
   \pgfutil@tempdima=0.4pt
   \advance\pgfutil@tempdima by.275\pgflinewidth%
   \pgfarrowsleftextend{+-\pgfutil@tempdima}
   \advance\pgfutil@tempdima by.5\pgflinewidth
   \pgfarrowsrightextend{+\pgfutil@tempdima}
 \else%
   \pgfutil@tempdima=\pgfgetarrowoptions{squarenew}%
   \divide\pgfutil@tempdima by 8%
   \pgfarrowsleftextend{+-7\pgfutil@tempdima}%
   \pgfarrowsrightextend{+1\pgfutil@tempdima}%
 \fi%
}
{
 \ifdim\pgfgetarrowoptions{squarenew}=-1pt%
   \pgfutil@tempdima=0.4pt%
   \advance\pgfutil@tempdima by.275\pgflinewidth%
   \pgfutil@tempdimb=0pt%
 \else%
   \pgfutil@tempdima=\pgfgetarrowoptions{squarenew}%   
   \divide\pgfutil@tempdima by 8%
   \pgfutil@tempdimb=0.5\pgflinewidth%
 \fi%
 \pgfsetdash{}{+0pt}
 \pgfsetroundjoin
 \pgfpathmoveto{\pgfpointadd{\pgfqpoint{1\pgfutil@tempdima}{4\pgfutil@tempdima}}
                            {\pgfqpoint{-\pgfutil@tempdimb}{-\pgfutil@tempdimb}}}
 \pgfpathlineto{\pgfpointadd{\pgfqpoint{-7\pgfutil@tempdima}{4\pgfutil@tempdima}}
                            {\pgfqpoint{\pgfutil@tempdimb}{-\pgfutil@tempdimb}}}
 \pgfpathlineto{\pgfpointadd{\pgfqpoint{-7\pgfutil@tempdima}{-4\pgfutil@tempdima}}
                            {\pgfqpoint{\pgfutil@tempdimb}{\pgfutil@tempdimb}}}
 \pgfpathlineto{\pgfpointadd{\pgfqpoint{1\pgfutil@tempdima}{-4\pgfutil@tempdima}}
                            {\pgfqpoint{-\pgfutil@tempdimb}{\pgfutil@tempdimb}}}
 \pgfpathclose
 \pgfusepathqfillstroke
}

% alternative stealth arrow
\pgfarrowsdeclare{stealthnew}{stealthnew}
{
  \ifdim\pgfgetarrowoptions{stealthnew}=-1pt%
    \pgfutil@tempdima=0.28pt%
    \pgfutil@tempdimb=\pgflinewidth%
    \ifdim\pgfinnerlinewidth>0pt%
      \pgfmathsetlength\pgfutil@tempdimb{.6\pgflinewidth-.4*\pgfinnerlinewidth}%
    \fi%
    \advance\pgfutil@tempdima by.3\pgfutil@tempdimb%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{stealthnew}%
    \divide\pgfutil@tempdima by 8%
  \fi%
  \pgfarrowsleftextend{+-3\pgfutil@tempdima}
  \pgfarrowsrightextend{+5\pgfutil@tempdima}
}
{
  \ifdim\pgfgetarrowoptions{stealthnew}=-1pt%
    \pgfutil@tempdima=0.28pt%
    \pgfutil@tempdimb=\pgflinewidth%
    \ifdim\pgfinnerlinewidth>0pt%
      \pgfmathsetlength\pgfutil@tempdimb{.6\pgflinewidth-.4*\pgfinnerlinewidth}%
    \fi%
    \advance\pgfutil@tempdima by.3\pgfutil@tempdimb%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{stealthnew}%
    \divide\pgfutil@tempdima by 8%
    \pgfsetlinewidth{0bp}%
  \fi%
  \pgfpathmoveto{\pgfqpoint{5\pgfutil@tempdima}{0pt}}
  \pgfpathlineto{\pgfqpoint{-3\pgfutil@tempdima}{4\pgfutil@tempdima}}
  \pgfpathlineto{\pgfpointorigin}
  \pgfpathlineto{\pgfqpoint{-3\pgfutil@tempdima}{-4\pgfutil@tempdima}}
  \pgfusepathqfill
}

% alternative stealth reversed arrow
\pgfarrowsdeclarereversed{stealthnew reversed}{stealthnew reversed}{stealthnew}{stealthnew}

% alternative to arrow
\pgfarrowsdeclare{tonew}{tonew}
{
  \ifdim\pgfgetarrowoptions{tonew}=-1pt%
    \pgfutil@tempdima=0.84pt%
    \advance\pgfutil@tempdima by1.3\pgflinewidth%
    \pgfutil@tempdimb=0.21pt%
    \advance\pgfutil@tempdimb by.625\pgflinewidth%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{tonew}%
    \pgfarrowsleftextend{+-0.8\pgfutil@tempdima}%
    \pgfarrowsrightextend{+0.2\pgfutil@tempdima}%
  \fi%
}
{
  \ifdim\pgfgetarrowoptions{tonew}=-1pt%
    \pgfutil@tempdima=0.28pt%
    \advance\pgfutil@tempdima by.3\pgflinewidth%
    \pgfutil@tempdimb=0pt,%
  \else%
    \pgfutil@tempdima=\pgfgetarrowoptions{tonew}%
    \multiply\pgfutil@tempdima by 100%
    \divide\pgfutil@tempdima by 375%
    \pgfutil@tempdimb=0.4\pgflinewidth%
  \fi%
  \pgfsetdash{}{+0pt}
  \pgfsetroundcap
  \pgfsetroundjoin
  \pgfpathmoveto{\pgfpointorigin}
  \pgflineto{\pgfpointadd{\pgfpoint{0.75\pgfutil@tempdima}{0bp}}
                         {\pgfqpoint{-2\pgfutil@tempdimb}{0bp}}}
  \pgfusepathqstroke
  \pgfsetlinewidth{0.8\pgflinewidth}
  \pgfpathmoveto{\pgfpointadd{\pgfqpoint{-3\pgfutil@tempdima}{4\pgfutil@tempdima}}
                             {\pgfqpoint{\pgfutil@tempdimb}{0bp}}}
  \pgfpathcurveto
  {\pgfpointadd{\pgfqpoint{-2.75\pgfutil@tempdima}{2.5\pgfutil@tempdima}}
               {\pgfqpoint{0.5\pgfutil@tempdimb}{0bp}}}
  {\pgfpointadd{\pgfqpoint{0pt}{0.25\pgfutil@tempdima}}
               {\pgfqpoint{-0.5\pgfutil@tempdimb}{0bp}}}
  {\pgfpointadd{\pgfqpoint{0.75\pgfutil@tempdima}{0pt}}
               {\pgfqpoint{-\pgfutil@tempdimb}{0bp}}}
  \pgfpathcurveto
  {\pgfpointadd{\pgfqpoint{0pt}{-0.25\pgfutil@tempdima}}
               {\pgfqpoint{-0.5\pgfutil@tempdimb}{0bp}}}
  {\pgfpointadd{\pgfqpoint{-2.75\pgfutil@tempdima}{-2.5\pgfutil@tempdima}}
               {\pgfqpoint{0.5\pgfutil@tempdimb}{0bp}}}
  {\pgfpointadd{\pgfqpoint{-3\pgfutil@tempdima}{-4\pgfutil@tempdima}}
               {\pgfqpoint{\pgfutil@tempdimb}{0bp}}}
  \pgfusepathqstroke
}

% alias alternative to arrow
\pgfarrowsdeclarealias{<new}{>new}{tonew}{tonew}

\makeatother

% tip length code
\pgfsetarrowoptions{latexnew}{-1pt}
\pgfsetarrowoptions{latex'new}{-1pt}
\pgfsetarrowoptions{onew}{-1pt}
\pgfsetarrowoptions{squarenew}{-1pt}
\pgfsetarrowoptions{stealthnew}{-1pt}
\pgfsetarrowoptions{tonew}{-1pt}
\pgfkeys{/tikz/.cd, arrowhead/.default=-1pt, arrowhead/.code={
  \pgfsetarrowoptions{latexnew}{#1},
  \pgfsetarrowoptions{latex'new}{#1},
  \pgfsetarrowoptions{onew}{#1},
  \pgfsetarrowoptions{squarenew}{#1},
  \pgfsetarrowoptions{stealthnew}{#1},
  \pgfsetarrowoptions{tonew}{#1},
}}

Answer 2

TikZ v3.0 (among other new features) includes a new arrows.meta library. It's probably inspired in Luigi's library and deprecates previous arrows and arrows.spaced libraries.

With this new library, it's very easy to define arrowhead parameters like length or width depending or not from line width.

\draw[-{Latex[length=3mm,width=5mm]}] (0,0)--(2,0);

will draw an arrow with a Latex arrowhead 3mm long and 5mm thick. All new arrows have changed their name which now starts with a capital letter to distinguish them from old names. Of course old names are still valid but they don't accept options as in previous example.

It's worth to read section "16 Arrows" in pgfmanual.

Some little examples:

\documentclass[tikz, border=2mm]{standalone}
\usetikzlibrary{arrows.meta}
\begin{document}
\begin{tikzpicture}
\draw[-latex] (0,0)--(2,0) node[right]{\emph{Old} latex arrow};
\draw[-Latex] (0,-.5)--(2,-.5) node[right]{\emph{new} Latex arrow};
\draw[-{Latex[length=3mm]}] (0,-1)--(2,-1) node[right]{with fixed length};
\draw[-{Latex[width=3mm]}] (0,-1.5)--(2,-1.5) node[right]{with fixed width};
\draw[-{Latex[length=5mm, width=2mm]}] (0,-2)--(2,-2) node[right]{widh fixed length and width};
\draw[-{Latex[red]}] (0,-2.5)--(2,-2.5) node[right]{with colorful tip};
\end{tikzpicture}
\end{document}

More examples about using arrows.meta:

• Skew an Arrow in a Tikz Diagram
• Custom arrow tip with `arrows.meta` TikZ library
• Adjusting the size of an arrow

Answer 3

Here is a sample of the outward-pointing triangular tips in PGF/TikZ 2.10 (see Section 24 "Arrow Tip Library" of the 2.10 manual):

\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{arrows}
\begin{document}
\newcounter{tmp}
\begin{tikzpicture}
\foreach \s in {latex,latex',stealth,triangle 90,triangle 45,angle 90, angle 60} {
    \stepcounter{tmp}
    \begin{scope}[yshift=-\thetmp cm]
        \node[anchor=west] (0,0) {\texttt{\s}};     
        \draw[arrows={\s-\s}] (3,0) --++ (1,0);
    \end{scope}
}
\end{tikzpicture}
\end{document}

If you want something more customized or more specific, read Section 82 "Arrow Tips" of the 2.10 manual of pgf. How much bigger and in what direction are you looking for?

Answer 4

Arrow sizes change with the line width. For example for thin and ultra thick lines:

If you do not want to have the whole line thicker, you can use the decoration library to add an enlarged arrow to your path. Here are two possibilities:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\begin{document}

\begin{tikzpicture}
   \draw[->] (0,0) -- (1,0);
   \draw[
    decoration={markings,mark=at position 1 with {\arrow[ultra thick]{>}}},
    postaction={decorate}
    ]
    (0,-0.5) -- (1,-0.5);
   \draw[
    decoration={markings,mark=at position 1 with {\arrow[scale=2]{>}}},
    postaction={decorate},
    shorten >=0.4pt
    ]
    (0,-1) -- (1,-1);
\end{tikzpicture}

\end{document}

Decorations are explained in chapter 31 "Decoration Library" of the TikZ 2.10 manual. The shorten >=0.4pt in the last example shortens the line by the default line width (0.4pt) so that it does not extend over the arrow tip (though the effect is nearly invisible for normal thin lines.

Answer 5

i just stumbled on the same problem, but those solutions didn’t fit my need. but i remembered postactions, so i came up with this:

\usetikzlibrary{arrows}
\begin{tikzpicture}
    \draw[
        -triangle 90,
        line width=4mm,
        postaction={draw, line width=1cm, shorten >=1cm, -}
    ] (0,0) -- (2,0);
\end{tikzpicture}

The idea is to choose a line width which yields the right arrow head, and then draw the line again with a fatter line.

Of course it only works when you need the line to be fatter than the head.

Answer 6

Here is a simple example of how to change all arrow heads in a document. On modern tikz versions, the arrows.meta tikz library provides a variety of customisable arrow heads and \tikzset can be used to set the default arrow style:

\documentclass{standalone}

\usepackage{tikz}

% add the following two lines to your document to get bigger arrows
\usetikzlibrary{arrows.meta}
\tikzset{>={Latex[width=3mm,length=3mm]}}

\begin{document}

\begin{tikzpicture}
\draw[->] (0,0) node[left]{from here} -- (3,1) node[right]{to here};
\end{tikzpicture}

\end{document}

You just need to add the marked two lines to your TeX file to get larger arrow heads. The size is controlled by the two parameters width and length. For the above example, the output is as follows:

There is a variety of arrow styles to choose from; the style used in the example is called Latex. Examples of relatively plain arrow styles include Stealth, Straight Barb and Computer Modern Rightarrow; any of these arrow styles can be used by replacing Latex in the above example with the corresponding style name. See the other answers and section 16.5 "Reference: Arrow Tips" of the TikZ and PGF manual for a complete list of available arrow styles.

Answer 7

Here's an alternative approach that replaces the arrow-rendering code with a customised version which allows for an extra scale factor to be applied (with only a little more work it would be possible to make it so that you could specify some formula to apply rather than just a linear scale factor and also provide different adjustments for the start and end). It does this by changing the line width just before the arrow is drawn and then restoring it afterwards. (It was derived from some code I had whose aim was to draw an arrow on a doubled line which used the inner line width instead of the outer one. I thought that was from a question here but can't find it now.)

This has the obvious advantage of not needing to modify every single arrow definition!

There are two macros to modify: the one that draws the arrows and the one that shortens the line.

\documentclass{article}
\usepackage{tikz}
%\url{http://tex.stackexchange.com/q/5461/86}

\makeatletter
\pgfkeys{
  /pgf/scalable arrows/.code={
    \let\pgf@add@arrows@as@needed=\pgf@add@scalable@arrows@as@needed
    \let\pgf@shorten@path@as@needed=\pgf@shorten@path@as@needed@for@scalable
  },
  /pgf/arrow scale factor/.initial=1,
}

\def\pgf@add@scalable@arrows@as@needed{%
  \pgfkeysgetvalue{/pgf/arrow scale factor}{\pgf@temp}%
  \let\pgf@restorelw=\pgfutil@empty
  \ifx\pgf@temp\pgfutil@empty
  \else
  \edef\pgf@restorelw{\noexpand\pgfsetlinewidth{\the\pgflinewidth}}%
  \pgfsetlinewidth{\pgf@temp\pgflinewidth}%
  \fi
  \ifx\pgf@startarrow\pgfutil@empty%
  \else%
    \pgflowlevelobj%
      {\pgftransformarrow{\pgfpointsecondonpath}{\pgfpointfirstonpath}}
      {\pgf@startarrow}%
  \fi%
  \ifx\pgf@endarrow\pgfutil@empty%
  \else%
    \pgflowlevelobj%
      {\pgftransformarrow{\pgfpointsecondlastonpath}{\pgfpointlastonpath}}
      {\pgf@endarrow}%
  \fi%
      \pgf@restorelw%
}

\def\pgf@shorten@path@as@needed@for@scalable{%
  \pgfkeysgetvalue{/pgf/arrow scale factor}{\pgf@temp}%
  \let\pgf@restorelw=\pgfutil@empty
  \ifx\pgf@temp\pgfutil@empty
  \else
  \edef\pgf@restorelw{\noexpand\pgfsetlinewidth{\the\pgflinewidth}}%
  \pgfsetlinewidth{\pgf@temp\pgflinewidth}%
  \fi
  \pgfprocesspathextractpoints{\pgf@arrowpath}%
  \let\pgf@arrow@next=\pgf@shorten@now%
  \ifx\pgf@shorten@start\pgfutil@empty%
    \ifx\pgf@shorten@end\pgfutil@empty%
      \ifdim\pgf@shorten@end@additional=0pt\relax%
        \ifdim\pgf@shorten@start@additional=0pt\relax%
          \let\pgf@arrow@next=\relax%
        \fi%
      \fi%
    \fi%
  \fi%
  \pgf@arrow@next%
  \pgf@restorelw%
}

\makeatother

\begin{document}
\begin{tikzpicture}
\begin{scope}[xshift=3cm]
\draw[green] (2,1) -- (2,-3);
\draw[line width=4pt,->] (0,0) -- (2,0);
\draw[line width=4pt,->] (0,-1) -- (2,-1);
\draw[line width=2pt,->] (0,-2) -- (2,-2);
\end{scope}
\begin{scope}[xshift=6cm]
\draw[green] (2,1) -- (2,-3);
\draw[line width=8pt,->] (0,0) -- (2,0);
\draw[line width=2pt,->] (0,-1) -- (2,-1);
\draw[line width=2pt,->] (0,-2) -- (2,-2);
\end{scope}
\begin{scope}[scalable arrows]
\draw[green] (2,1) -- (2,-3);
\draw[arrow scale factor=2,line width=4pt,->] (0,0) -- (2,0);
\draw[arrow scale factor=.5,line width=4pt,->] (0,-1) -- (2,-1);
\draw[line width=2pt,->] (0,-2) -- (2,-2);
\end{scope}
\end{tikzpicture}
\end{document}

The first column has the scalable arrows. The second shows what they would look like with no scaling, the third shows what they would look like with the line width to get that size arrow naturally. The green line is to show that the shortening is correctly applied.

Answer 8

I feel your pain there. The regular arrow head is almost invisible. You can declare your own arrow head using \pgfarrowsdeclare. You can modify the example below using commands from chapters 71 and 74 of the pgf manual. This is a simpler version of the example from chapter 74, which I latexified a bit while I was at it. Hope that helps. The idea behind declaring your own length is that it's going to scale with line thickness when you do it this way.

\newlength{\arrowsize}  
\pgfarrowsdeclare{biggertip}{biggertip}{  
  \setlength{\arrowsize}{0.4pt}  
  \addtolength{\arrowsize}{.5\pgflinewidth}  
  \pgfarrowsrightextend{0}  
  \pgfarrowsleftextend{-5\arrowsize}  
}{  
  \setlength{\arrowsize}{0.4pt}  
  \addtolength{\arrowsize}{.5\pgflinewidth}  
  \pgfpathmoveto{\pgfpoint{-5\arrowsize}{4\arrowsize}}  
  \pgfpathlineto{\pgfpointorigin}  
  \pgfpathlineto{\pgfpoint{-5\arrowsize}{-4\arrowsize}}  
  \pgfusepathqstroke  
}  

To use your arrow afterwards, say something like

\draw[-biggertip] (0,0) -- (0,1);