You might be better off increasing the overall text size for your forum.
AdminCP > Styles & Templates > Style Manager > Main CSS
Increase the "Font-size" numbers to fit your needs.
OR:
If you still just want to change it in the postbit your best bet would be to edit the postbit(_legacy) template.
AdminCP > Styles & Templates > Style Manger > Edit Templates > Postbit Templates
Find [minicode]$post[message][/minicode] and wrap around it as such:
Code:
<span style="font-size:14px">
$post[message]
</span>
Change 14 to whatever size you feel suitable.