TeX does not load all possible fonts, so some
things like \tiny\tt may fail, defaulting
to \tiny\rm, which can really ruin things if you
were depending on fixed-width fonts.

In the preamble, add a line that looks like

\makeatletter	 % Treat @ as a letter, allowing us to do \@ptscale
\font\figfont  = cmtt10   \@ptscale5 % typewriter
\makeatother     % Back to hiding \@xxxx macros from the user.

To set \figfont to a seven-point fixed-width font.

You could also do '\font\fivtt=cmtt10\@ptscale5'
which will make it available for whatever size 
command (e.g., \scriptsize or \tiny) looks for this
size of font.  Note that changing document style will
change what font sizes are searched for, so for a figure,
setting it to a specific font size is more appropriate 
than relying on \scriptsize and so forth.

You can also take a look at averbatim.sty.

Additional example variants of this line can be found
in /usr/local/TeX/lib/inputs/lfonts.tex.orig.