tikz customized shape too big

Question

I try to create a customize shape with tikzpicture within 1 unit, but when I use it, it's almost 2 unit size.

\documentclass[border=1pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
  \definecolor{bakcolor}{RGB}{220,150,100}
  \newsavebox\myheadphone
  \begin{lrbox}{\myheadphone}
    \begin{tikzpicture}
      \def\d{5pt}
      \draw[draw=none,fill={rgb,255:red,220; green,150; blue,100}] (0,0) circle (1);
      \draw[draw=white,line width=\d,line cap=round] (-40:1) arc (-40:220:1);
      \draw[draw=white,fill=white,line width=\d,rounded corners=2pt] (-40:1) rectangle ++(-0.3,-0.6);
      \draw[draw=white,fill=white,line width=\d,rounded corners=2pt] (220:1) rectangle ++(0.3,-0.6);
    \end{tikzpicture}
  \end{lrbox}
  \draw[help lines,] (-2, -2) grid[step={(1,1)}] (2, 2);
  \node[draw] (A) {A};
  \node[circle,fill=bakcolor,right=1cm of A,minimum size=4] (B) {\usebox\myheadphone};
  \draw[->] (A) -- (B);
\end{tikzpicture}
\end{document}

Output:

Obviously:

1. The border of the customized shape too wide.
2. The whole object too big. the minimum size keyword does't work.

Answer 1

What you've done is to first make a shape that in itself has a radius slightly larger than 1cm, then you put this shape in a rectangular box, and finally you put that box in a circular node. Nodes have an inner sep, a padding between the content and the border. As a result, the node you've put the shape in becomes larger than 2cm across. And as marmot rightly says, the minimum size defines a lower bound for the node size, if the content of the node is larger, the node becomes larger. That said, you've set minimum size=4, without a unit, and in this case the default unit is pt, I believe.

Anyways, if you want the headphone symbol to fit within a circle of radius 1cm, reduce the size of it. You might consider using a pic instead of a savebox though, see example below. Left is a savebox, right is a pic.

The savebox is inherently rectangular, so even if you set the shape of the node containing it to circle, the arrow still wouldn't reach the shape. You can circumvent that problem with a negative inner sep though, try \node [inner sep=-8.5pt,circle] (C) at (-2,0) {\usebox\myheadphone};.

\documentclass[border=1pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\definecolor{bakcolor}{RGB}{220,150,100}

\tikzset{
  headphone/.pic={
      \def\d{5pt}
      \node[draw=none,fill=bakcolor,minimum size=2cm,circle] (-outline) at (0,0){};
      \begin{scope}[yshift=\d/2]
      \draw[draw=white,line width=\d,line cap=round] (-40:0.6) arc[start angle=-40,end angle=220,radius=0.6];
      \filldraw[white,line width=\d,rounded corners=2pt] (-40:0.6) rectangle ++(-0.2,-0.35);
      \filldraw[white,line width=\d,rounded corners=2pt] (220:0.6) rectangle ++(0.2,-0.35);
      \end{scope}
  }
}

\begin{document}
\begin{tikzpicture}
\newsavebox\myheadphone
  \begin{lrbox}{\myheadphone}
    \begin{tikzpicture}
      \def\d{5pt}
      \draw[draw=none,fill={rgb,255:red,220; green,150; blue,100}] (0,0) circle (1);
      \begin{scope}[yshift=\d/2]
      \draw[draw=white,line width=\d,line cap=round] (-40:0.6) arc[start angle=-40,end angle=220,radius=0.6];
      \filldraw[white,line width=\d,rounded corners=2pt] (-40:0.6) rectangle ++(-0.2,-0.35);
      \filldraw[white,line width=\d,rounded corners=2pt] (220:0.6) rectangle ++(0.2,-0.35);
      \end{scope}
    \end{tikzpicture}
  \end{lrbox}
  \draw[help lines,] (-2, -2) grid[step={(1,1)}] (2, 2);
  \node[draw] (A) at (0,-2){A};

  \path (A) ++(2,2) pic (B) {headphone};

  \node [inner sep=0pt] (C) at (-2,0) {\usebox\myheadphone};

  \draw[->] (A) -- (B-outline);
  \draw[->] (A) -- (C);
\end{tikzpicture}
\end{document}

Here is a scalable version, where the scale parameter can be added as an argument to the pic, e.g. \pic {headphone=2}; to make it twice the size.

\documentclass[border=1pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\definecolor{bakcolor}{RGB}{220,150,100}

\tikzset{
  pics/headphone/.style={
    code={
      \def\d{5pt}
      \node[draw=none,fill=bakcolor,minimum size=2cm*#1,circle] (-outline) at (0,0){};
      \begin{scope}[yshift=\d/2*#1]
      \draw[draw=white,line width=\d*#1,line cap=round] (-40:0.6*#1) arc[start angle=-40,end angle=220,radius=0.6*#1];
      \filldraw[white,line width=\d*#1,rounded corners=2pt] (-40:0.6*#1) rectangle ++(-0.2,-0.35*#1);
      \filldraw[white,line width=\d*#1,rounded corners=2pt] (220:0.6*#1) rectangle ++(0.2,-0.35*#1);
      \end{scope}
 }},
 pics/headphone/.default=1,
}

\begin{document}
\begin{tikzpicture}
  \pic {headphone};
  \pic at (3,0) {headphone=2};
\end{tikzpicture}
\end{document}

Answer 2

I do not understand why you expect that that the minimum size option will make the (very nice) picture smaller. You can make it any size by using the scale option instead. For instance

\documentclass[border=1pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
  \definecolor{bakcolor}{RGB}{220,150,100}
  \newsavebox\myheadphone
  \begin{lrbox}{\myheadphone}
    \begin{tikzpicture}
      \def\d{5pt}
      \draw[draw=none,fill={rgb,255:red,220; green,150; blue,100}] (0,0) circle (1);
      \draw[draw=white,line width=\d,line cap=round] (-40:1) arc (-40:220:1);
      \draw[draw=white,fill=white,line width=\d,rounded corners=2pt] (-40:1) rectangle ++(-0.3,-0.6);
      \draw[draw=white,fill=white,line width=\d,rounded corners=2pt] (220:1) rectangle ++(0.3,-0.6);
    \end{tikzpicture}
  \end{lrbox}
  \draw[help lines,] (-2, -2) grid[step={(1,1)}] (2, 2);
  \node[draw] (A) {A};
  \node[circle,fill=bakcolor,right=1cm of A,scale=0.4] (B) {\usebox\myheadphone};
  \draw[->] (A) -- (B);
\end{tikzpicture}
\end{document}

yields

EDIT: After the comment, I believe to understand your question better. If you want to have a fixed size, you may want to steal this nice macro. The complete code is

\documentclass[border=1pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\usepackage{environ}
\makeatletter
\newsavebox{\measure@tikzpicture}
\NewEnviron{scaletikzpicturetowidth}[1]{%
  \def\tikz@width{#1}%
  \def\tikzscale{1}\begin{lrbox}{\measure@tikzpicture}%
  \BODY
  \end{lrbox}%
  \pgfmathparse{#1/\wd\measure@tikzpicture}%
  \edef\tikzscale{\pgfmathresult}%
  \BODY
}
\makeatother
\begin{document}
\begin{tikzpicture}
  \definecolor{bakcolor}{RGB}{220,150,100}
  \newsavebox\myheadphone
  \begin{lrbox}{\myheadphone}
    \begin{scaletikzpicturetowidth}{1cm}
    \begin{tikzpicture}[scale=\tikzscale]
      \def\d{5pt}
      \draw[draw=none,fill={rgb,255:red,220; green,150; blue,100}] (0,0) circle (1);
      \draw[draw=white,line width=\d,line cap=round] (-40:1) arc (-40:220:1);
      \draw[draw=white,fill=white,line width=\d,rounded corners=2pt] (-40:1) rectangle ++(-0.3,-0.6);
      \draw[draw=white,fill=white,line width=\d,rounded corners=2pt] (220:1) rectangle ++(0.3,-0.6);
    \end{tikzpicture}
    \end{scaletikzpicturetowidth}
  \end{lrbox}
  \draw[help lines,] (-2, -2) grid[step={(1,1)}] (2, 2);
  \node[draw] (A) {A};
  \node[circle,fill=bakcolor,right=1cm of A] (B) {\usebox\myheadphone};
  \draw[->] (A) -- (B);
\end{tikzpicture}
\end{document}

The problem I have with this is that your nice headphone code involves some absolute (or explicit) lengths (such as 5pt). Therefore, this method of scaling the picture does not always yield a nice result.