How to get an even smaller font?

A quick-and-dirty possibility for converting almost anything to a different size is to use graphicx's \resizebox or \scalebox. In your instance, if line-breaking is not a consideration, you can use

\usepackage{graphicx}% http://ctan.org/pkg/graphicx
%...
Normal {\tiny tiny} \scalebox{.2}{supertiny}

The above view is zoomed to 400%. You can modify the scaling factor .2 to whatever you're interested in.

The size changing commands don't have a cumulative effect, so you simply get \tiny. Recall that the size chosen would be 5pt if the main size for the document is 10pt. Under 5pt characters are barely legible.

Here's an example:

\documentclass{article}
\usepackage{lmodern}
\begin{document}

Normal {\tiny Tiny} {\fontsize{2.5}{4}\selectfont Supertiny}

\end{document}

Notice that the image here is magnified. Without lmodern or a package that chooses a scalable font you wouldn't get "supertiny", unless you load the fix-cm package.

If you do not need large font sizes, too, the easiest way is to use a document class that supports an arbitrary size for the main font, such as scrartcl, the article class from the KOMA-Script bundle:

\documentclass[fontsize=6pt]{scrartcl}
\usepackage{lmodern}
\begin{document}

\rule{2pt}{12pt}12pt\rule{2pt}{12pt} Normal 6pt {\tiny Tiny 3pt} 

\end{document}

The height of the bars represents 12pt.