[Adrian Bell]

I've never had a problem with font sizes in Thunderbird. The interface uses the same fonts as every other program I have (Win7). However have you looked here?
http://kb.mozillazine.org/Font_settings_in_Thunderbird
http://kb.mozillazine.org/Pane_and_menu_fonts
http://kb.mozillazine.org/Bad_Eyesight_-_Thunderbird

http://www.rossde.com/ThemeFontSizeC...ire/index.html

If you have any questions about Thunderbird this is the best place to ask:-
http://forums.mozillazine.org/index.php