In your template, this is what it should be:
BODY - FONT-SIZE: 12px
TD - FONT-SIZE: 12px
INPUT - FONT-SIZE: 9pt
TEXTAREA - FONT-SIZE: 9pt
SELECT - FONT-SIZE: 7pt
.windowbg - FONT-SIZE: 12px
.windowbg2 - FONT-SIZE: 12px
.windowbg3 - FONT-SIZE: 12px
.text1 - FONT-SIZE: 12px
.catbg - FONT-SIZE: 13px
.bordercolor - FONT-SIZE: 12px
.quote - FONT-SIZE: 10px
.code - FONT-SIZE: 10px
If you go into you admin center, you will find a place where it says "Edit your forum template", that makes it all very easy to change.
Hope this will help you
