I been doing searches for a while trying to figure out how to fix this problem after downloading AIM 6.5...
The problem was that my default font (8) looked like its 12 all the time, even though everyone else saw it as 8...
and it would soon go up to 12 again to everyone else anyway, usually after making a smiley face at someone...
I realized it has to do with Internet Explorer (I use IE 7 so I'm not sure if its the same with older versions)
This is what I did (try it if you have this problem)
Open your IE
Go to Tools>Internet Options
Click the Accessibility button at the bottom of the General tab
Uncheck all that are in the formatting section... the ones that say "Ignore yada yada"
Make sure you Okay out of it after you do so
then go to the Advanced tab (in Tools>Internet Options)
Uncheck all that are in the first section "Accessibility"
Press Okay
Exit out of your current chat box and reopen
Hope this helps some of you out there, too