Set the size of pgf picture?

Maybe simply \scalebox:

\documentclass{article}

\usepackage{fontspec} % enagles loading of OpenType fonts
\usepackage{polyglossia} % support for languages

% fonts:
\defaultfontfeatures{Scale=MatchLowercase,Mapping=tex-text} % without this XeLaTeX won't turn "--" into dashes
\setmainfont{DejaVu Sans}
\setsansfont{DejaVu Sans}
\setmonofont{DejaVu Sans Mono}

% pgf:
\usepackage{pgf}

\begin{document}

\scalebox{0.6}{\input{m1.pgf}}

\end{document}

You can first draw a phantom to determine the sizes, compute the scaling factor(s) and draw the image again:

Code

\documentclass[a5paper]{scrreprt}
\usepackage{xifthen}
\usepackage{environ}
\usepackage{tikz}

\pgfmathsetmacro{\maximumpicturewidth}{10}% max width in cm
\pgfmathsetmacro{\maximumpictureheight}{5}% max height in cm

\newcommand{\getxyscale}
{   \path (current bounding box.south west);
  \pgfgetlastxy{\xsw}{\ysw}
  \path (current bounding box.north east);
  \pgfgetlastxy{\xne}{\yne}
  \pgfmathsetlengthmacro{\picwidth}{\xne-\xsw}
  \pgfmathsetlengthmacro{\picheight}{\yne-\ysw}
  \pgfmathsetlengthmacro{\maxpicwidth}{\maximumpicturewidth*28.453}
  \pgfmathsetlengthmacro{\maxpicheight}{\maximumpictureheight*28.453}
  \pgfmathsetmacro{\xscale}{\maxpicwidth/\picwidth}
  \pgfmathsetmacro{\yscale}{\maxpicheight/\picheight}
  \xdef\xscalefactor{\xscale}
  \xdef\yscalefactor{\yscale}
}

\NewEnviron{autoscaletikz}%
{\noindent\hphantom{\vphantom{\begin{tikzpicture}\BODY\getxyscale\end{tikzpicture}}}%
\noindent\begin{tikzpicture}[xscale=\xscalefactor,yscale=\yscalefactor]\BODY\end{tikzpicture}}

\begin{document}

\begin{autoscaletikz}
    \fill[red] (0,0) circle (5 and 1);
    \fill[draw=black,left color=blue, right color=orange] (0.5,0) rectangle (4,2);
    \draw (2,-1) rectangle (18,1);
    \draw (4,-0.5)
    \foreach \x in {1,...,12}
    {   arc ({mod(\x,4)*90}:{(mod(\x,4)+1)*90}:{\x/10})
    }   ;
\end{autoscaletikz}

\end{document}

If you want to keep the aspect ratio, simply change the environment:

\NewEnviron{autoscaletikz}%
{\noindent\hphantom{\vphantom{\begin{tikzpicture}\BODY\getxyscale\end{tikzpicture}}}%
\noindent\begin{tikzpicture}[xscale=\xscalefactor,yscale=\xscalefactor]\BODY\end{tikzpicture}}

Result