How do you change the sizes of the tags?

if you mean the tag cloud, that changes automatically based on the number of occurances of each tag. i.e. if there are 3 post with 'boo' and 200 tags with 'hoo', boo will be small and hoo will be large

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.”)