Font sizing is a nightmare, and none of the hacks and workarounds are ideal.
One thing I would suggest though to help is to ditch the Trebuchet font. It looks like you've used the standard Moodle selection of:
"Trebuchet MS", Verdana, Arial, Helvetica, sans-serif;
I would recommend:
"Lucida Grande", Verdana, "Bitstream Vera Sans", Geneva, Lucida, Arial, Helvetica, sans-serif;
This means most users (Windows/IE) will see Verdana, which is better-looking (ihmo) and more legible on screen than Trebuchet, especially at smaller sizes and on systems without anti-aliasing.
The other options ensure that Mac and Linux users get beautiful, readable fonts that are suited to their platforms too.
Martin, if you're reading this, do you think we could add this list of fonts to the standard themes? (optionally with Trebuchet MS in there somewhere too).