Daniel, the category thumbs in vb4 have the same css that the forum icons have--I have not found a way to change just the category icons without making a mess of the css in photopost.
This becomes especially difficult if you are using a non-stock
vb-theme.
maybe you'll have better luck with it, check the stylevars for forumicon_size and forumiconbox_size. You'll still need to change them where chuck was talking about as well, just make sure you do it in the area for vb4 in pp-inc.