Ok, I think I found a rather good explanation here: LaTeX Community • View topic - Font Sizes:
size10.clo defines
\newcommand\Huge{\@setfontsize\Huge\@xxvpt{30}}
The macro \@xxvpt, which looks like roman representation of arabic 25pt, is defined in latex.ltx:
\def\@xxvpt{24.88}
Similar for the other values.
... and so, if one checks the size8.clo - it similarly defines all of the parameters, more or less, explicitly - instead of relatively (which the original post assumes).
So probably the plain Latex way would be to copy the size8.clo into a new file, and then change all parameters there (and then load the new size / changed file)... but that's gonna be some work :)
If anyone's got anything easier, please post :)
For the sizes, you can use the current bounding box together with \pgfgetlastxy:
\documentclass{standalone}
\usepackage{tikz}
\newcommand*{\ExtractCoordinate}[3]{\path (#1); \pgfgetlastxy{#2}{#3};}%
\newdimen\tlx
\newdimen\tlx
\newdimen\brx
\newdimen\bry
\begin{document}
\begin{tikzpicture}
\draw (-3,-1) rectangle (3,1) (-2,-2) rectangle (2,2);
\foreach \x in {north west,south west,south east,north east}
{ \fill[red] (current bounding box.\x) circle (0.01);
}
\ExtractCoordinate{current bounding box.north west}{\tlx}{\tly}
\ExtractCoordinate{current bounding box.south east}{\brx}{\bry}
\pgfmathsetmacro{\myheight}{(+\tly-\bry)/28.452755}
\pgfmathsetmacro{\mywidth}{(-\tlx+\brx)/28.452755}
\node[below right] at (-2,1) {height: \myheight\ cm};
\node[above left] at (2,-1) {width: \mywidth\ cm};
\end{tikzpicture}
\end{document}
Edit 1: Ah, now I get it. Here is a still not automatic version. You set two commands in each tikzpicture:
\maximumdimensions{4}{3}, which is the desired width and height, at the beginning of the picture.
\getscalingfactors which will draw nodes with a recommended scaling factor in the middle of your picture if the image if to high/wide:
\documentclass{scrartcl}
\usepackage{tikz}
\usepackage{lipsum}
\usepackage{xifthen}
\newcommand*{\ExtractCoordinate}[3]{\path (#1); \pgfgetlastxy{#2}{#3};}%
\newdimen\tlx
\newdimen\tlx
\newdimen\brx
\newdimen\bry
\def\xscalefactor{}
\def\yscalefactor{}
\parindent0mm
\newcommand{\getscalingfactors}{%
\ExtractCoordinate{current bounding box.north west}{\tlx}{\tly}
\ExtractCoordinate{current bounding box.south east}{\brx}{\bry}
\pgfmathsetmacro{\myheight}{(+\tly-\bry)/28.452755}
\pgfmathsetmacro{\mywidth}{(-\tlx+\brx)/28.452755}
\pgfmathsetmacro{\xsf}{\maxwidth/\mywidth}
\pgfmathsetmacro{\ysf}{\maxheight/\myheight}
\global\edef\xscalefactor{\xsf}
\global\edef\yscalefactor{\ysf}
\pgfmathtruncatemacro{\xsfc}{\xsf*10000}
\pgfmathtruncatemacro{\ysfc}{\ysf*10000}
\ifthenelse{\xsfc > 10000}{}{\node[fill=white,above] at (current bounding box.center) {x-s: \xsf};}
\ifthenelse{\ysfc > 10000}{}{\node[fill=white,below] at (current bounding box.center) {y-s: \ysf};}
}
\newcommand{\maximumdimensions}[2]{% width, height
\global\edef\maxwidth{#1} % in cm
\global\edef\maxheight{#2} % in cm
}
\begin{document}
\begin{tikzpicture}[scale=1]
\maximumdimensions{4}{3}
\draw[fill=orange] (-3,-1) rectangle (3,1) (-2,-2) rectangle (2,2);
\getscalingfactors
\end{tikzpicture}
\lipsum[1]
\begin{tikzpicture}[scale=1]
\maximumdimensions{4}{3}
\draw[fill=orange!50!gray] (0,0) circle (3);
\draw[fill=red!50!gray] (3,1) circle (1);
\draw[fill=blue!50!gray] (-2,2) circle (1);
\draw[fill=green!50!gray] (-3,-2) circle (1);
\draw[fill=yellow!50!gray] (2,-1) circle (1);
\getscalingfactors
\end{tikzpicture}
\end{document}
If you scale the picture down enough, the nodes inside the picture vanish:
\begin{tikzpicture}[scale=0.66] and \begin{tikzpicture}[scale=0.497]
Best Answer
To my surprise, the first or the second optional arguments of
\chemfigdo not take the optionfont=\Large(which is atikzoption for node font), contrary to what manual states. Here are the two methods mentioned by cgnieder along with a scalebox version (very hacky).