So yeah, I can't figure out how to set the size of a block of interface text. I mean, iface.text.textLarge does change the size, but only larger (i think), and I'm trying to make it smaller.

The interface.xml file doesn't appear to have it either, unless I'm missing something.
Would I just have to create a whole new font?
Well, you could just scale down the font. I don't think there's a good way to change the font size currently. :|
You would have to make your own font bitmap, it's not easy though to get it to look good and aligned correctly, it's 'trial and error'!
Couldn't Brian just add support for font files? That way, we won't have to make bitmaps and all that, and using a lot of different sizes (not just small and large) would be possible. I think this should be considered at least...
I whole-heartedly agree. :P
I was thinking about this and wondering if Web kit
http://webkit.org/ could be added to the core, this could possibly add a layer to the hud allowing html style rendering for text and graphics. Just an idea...
That post belongs in the Support section...