The forum currently assumes that you've already used your image editing program of choice to properly size the logo. You have two options:
1. Use an image editing program to re-size the logo appropriately (a.k.a. the better option)
2. Edit inc_header.asp around line line 280 find getCurrentIcon(strTitleImage & "||",strForumTitle,"")
and add the parts in red: getCurrentIcon(strTitleImage & "|width|height",strForumTitle,"")
(a.k.a. not the best option)
Option one will improve your page load time as visitors won't have to download a big freaking image.