How to specify font size less than 10pt (or more than 12pt)?

• extsizes classes offer a base font size between 8 and 20 pt, you may choose the extarticle class

• KOMA-Script classes support freely customizable base font sizes and you may specify them by any TeX length unit like pt, bp or mm, so you might use scrartcl.

I recommend to use a KOMA-Script class.

Instead of switching to a KOMA-Script class, you may also use the scrextend package (part of KOMA-Script) and its \changefontsizes macro. It features an optional argument to change \baselineskip (the default is 1.2 * fontsize). For details see section 3.5 of the KOMA-Script manual (the relevant content also applies to scrextend, as mentioned in section 9.5 of the manual). EDIT: Make sure to use a scalable font (e.g. Latin Modern instead of Computer Modern).

\documentclass{article}

\usepackage{lmodern}

\usepackage{scrextend}
\changefontsizes[20pt]{16pt}

\usepackage{lipsum}

\begin{document}

\lipsum[1]

\end{document}

It is a semantically better practice to define a new font size which is a size less than tiny, rather than use \fontsize, by using:

\makeatletter
  \newcommand\tinyv{\@setfontsize\tinyv{4pt}{6}}
\makeatother

I called, it tinyv (v being the size in roman numerals), but you can call it anything you wish.

You can also change any of the others this way, by using renewcommand. You can do this in the class file you are using, in which case you do not need the apocryphal \makeatletter and makeatother.

\renewcommand\scriptsize{@setfontsize\scriptsize{5}{7}}