I hacked up this little plugin that lets you change the size of the tag cloud without hacking any core files. (And, yeah, that's all it does.)
It does, however, require you to change some code in your template to call the function.
(I took the code from template-functions.php line 1150 or so, the tag_heat_map.)
See it here or download it here.
You can comment about it on my site if you wish. My Release Annoncement.