What do you mean exactly by saying "all documents"? (Font size is a matter of documentclasses.) Is your point maybe restricted to the issue of handling fonts differently in text and math mode?
– RubenNov 04 '13 at 20:55
thanks for answers. what i need is like "text=13pt", "math=12pt" in a document.
– mertNov 04 '13 at 21:23
In addition to @MarcoDaniel's answer, you can use \renewcommand{\normalsize}{\fontsize{13}{15}\selectfont} to make the body text size 13pt (with 15pt \baselineskip), but note that this will NOT affect heading sizes, footnote sizes, or commands like \large, etc.
– ChrisSNov 05 '13 at 02:01
\renewcommand{\normalsize}{\fontsize{13}{15}\selectfont}to make the body text size 13pt (with 15pt\baselineskip), but note that this will NOT affect heading sizes, footnote sizes, or commands like\large, etc. – ChrisS Nov 05 '13 at 02:01