i had to do this at one time ages ago.
you could try:
find in postbit template:
<!-- message -->
<div id="post_message_$post[postid]">
$ad_location[ad_showthread_firstpost_start]
$post[message]
</div>
<!-- / message -->
remove and replace with this:
<!-- message -->
<div id="post_message_$post[postid]" class="vb_postbit">
$ad_location[ad_showthread_firstpost_start]
$post[message]
</div>
<!-- / message -->
next, add in Additional CSS of that style:
.vb_postbit {
font-size: 14px;
font-family: verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif;
}
change 14px to 13px or what ever. hope it works. ah and you may not need font-family: in css but try it.