Go to your Admin CP.
Styles & Templates ==>
Search in Templates
Type in (or copy/pasta):
Code:
.forumbit_post .forumrow .forumlastpost {
Double-click on "forumbits.css" if it pops up in the results, and press "Find" to be scrolled to where your search text is.
Right there, there should be a font-size declaration. Change that as necessary.
NOTE: if it's a StyleVar (something like
{vb:stylevar mid_fontSize}), then you don't need to edit the template at all--just go change the correct StyleVar for the style.