[Bluefin221]

Sorted it myself, for future reference:

Add this to the "Additional CSS Definitions" box in MainCSS:

.text_size_attribute {font-size: 12pt;}

## Edit the postbit and postbit_legacy templates. Find this:

<!-- message -->
<div id="post_message_$post[postid]">
$ad_location[ad_showthread_firstpost_start]
$post[message]
</div>
<!-- / message -->

## and change it to this: (add the div class)

<!-- message -->
<div
class="text_size_attribute" id="post_message_$post[postid]">
$ad_location[ad_showthread_firstpost_start]
$post[message]
</div>
<!-- / message -->