Is there an TikZ command to provide the height and width of the picture? [duplicate]

Question

I've found the following How can I compute the distance between two coordinates in TikZ?. So, I know how to find the size of a TikZ picture as follows:

\documentclass{article}
\usepackage[margin=0.5in]{geometry}
\usepackage{fp}
\usepackage{tikz}
\usetikzlibrary{calc}

\makeatletter
\def\calcLength(#1,#2)#3{%%
  \pgfpointdiff{\pgfpointanchor{#1}{center}}%%
               {\pgfpointanchor{#2}{center}}%%
  \pgf@xa=\pgf@x%%
  \pgf@ya=\pgf@y%%
  \FPeval\@temp@a{\pgfmath@tonumber{\pgf@xa}}%%
  \FPeval\@temp@b{\pgfmath@tonumber{\pgf@ya}}%%
  \FPeval\@temp@sum{(\@temp@a*\@temp@a+\@temp@b*\@temp@b)}%%
  \FProot{\FPMathLen}{\@temp@sum}{2}%%
  \FPround\FPMathLen\FPMathLen5\relax
  \global\expandafter\edef\csname #3\endcsname{\FPMathLen pt}%%
  }
\makeatother

\setlength{\parindent}{0pt}

\begin{document}


\begin{tikzpicture}[baseline=(B)]%%
  \coordinate (A) at (0,0);
  \coordinate (B) at (10,3);
  \draw (A) -- (B);

  \coordinate (PUL) at ($(current bounding box.north west)+(-1ex,1ex)$);
  \coordinate (PLR) at ($(current bounding box.south east)+(1ex,-1ex)$);
  \coordinate (PUR) at (PUL-|PLR);
  \coordinate (PLL) at (PUL|-PLR);
  \draw (PUL) rectangle (PLR);

  \calcLength(PUL,PUR){mywidth}%%
  \calcLength(PUL,PLL){myheight}%%

\end{tikzpicture}%%

Here's a rule the width of the picture:

\rule{\mywidth}{4pt}

Here's a rule the height of the picture:

\rule{4pt}{\myheight}

\end{document}

What I'm curious about is whether isn't an easier way to go about this since all I'm interested in is the dimensions of the finished picture.