It's simple actually. Go to your Admin Panel, Style Manager - Your Style - Edit Templates - Postbit Templates - Pick the one you use, "postbit" or "postbit_legacy".

Find...

PHP Code: 
        <!-- message -->
        <
div id="post_message_$post[postid]">
            
$ad_location[ad_showthread_firstpost_start]
            
$post[message]
        </
div>
        <!-- / 
message --> 
Now you're going to do a simple change to its div.

Change from this:

PHP Code: 
<div id="post_message_$post[postid]"
To this:
PHP Code: 
<div id="post_message_$post[postid]style="font-size: 15px"
Hit save.. and voila!

P.S. You can adjust the font size value to the one you prefer...