Check your "forumdisplaybit" template. The code that displays the icon is
Code:
<td bgcolor="{secondaltcolor}">$thread[icon]</td>
But I don't see any settings for the the size of the icons. Meaning that it should just diplay the icon the actual size that it is. So if your icons are 30 pixels wide, then they should all display just like your "Post" icon in the screenshot.
Somebody correct me if I'm wrong.