Font size, with the New Font Selection System, can be selected in the following way:
\fontsize{size}{leading}\selectfont
where size is the point size you want (say, 10pt) and leading is the leading (space between lines) you want (say, 12pt).
This does, as mentioned, require scalable fonts, or at least font definition files which permit dynamic generation of the appropriately-sized bitmap fonts. If you say \usepackage{lmodern} you'll get the fully-scalable Latin Modern fonts, which are near-duplicates of Computer Modern; if you really want to use Computer Modern, you can say \RequirePackage{fix-cm} before your \documentclass command to get better font definition files for Computer Modern.
As David Carlisle says, though, shrinking your table to such small sizes to make it fit is generally a bad idea, as it impedes readability and clashes with your normal-size text. Better options are reduce spacing between columns, or even setting the table in landscape mode on its own page; see the lscape package.
If you're not already using it, look at the booktabs package, as well, which makes tables created in LaTeX worlds better.
Size commands do not work in math mode. However (some) size commands set up math so if you enter a new math expression from text while the size change is in force, you get math of a matching size. Internally array is an \halign in which each cell is surrounded by $...$ so in fact using array is like using \mbox{empty text $ matrix cell$} so the entries pick up the size change more or less by accident.
\documentclass{scrbook}
\usepackage{amsmath}
\begin{document}
$M_{\begin{smallmatrix}2\\2\end{smallmatrix}2A}$
\end{document}
Best Answer
\smalland\tinyare text font macros (just like\large,\huge, ...). You most likely received the following font warnings in your.logfile:Inside math mode, in order to use a different (smaller) font, you could/should use
\scriptstyleor\scriptscriptstyle:\scriptstyledenotes the font size of super-/subscripts, while\scriptscriptstyledenotes the font size for super-/subscripts of super-/subscripts. Thereafter (higher scripting), the font size remains at\scriptscriptsize. SeeThat's why there's a suggestion to use
\scalebox(and possibly some height adjustment using\raisebox) - it allows you to have a little more variation in the fonts in smaller/larger sized.