You into your client & extract the "local" file...
Then open the "local_game.txt" and scroll down to the line 749
There you'll find this paragraph
Code:
UI_DEF_FONT Tahoma:12
UI_DEF_FONT_LARGE Tahoma:14
UI_DEF_FONT_SMALL Tahoma:9
And there you can change the font and the size, too.
hope that this was helpful..