Sorry, I wanted to be able to adjust the max. size... but since then I figured it out.
- Open the file /wp-includes/category-template.php.
- Scroll down to the line that begins function wp_tag_cloud. If you’re using WordPress 2.7 and haven’t made any other changes to this file, you should find it at line 552. (You can always do a search for it; I found the general area by searching for “tag cloud.”)