I had a problem after installing OpenSUSE 11.1 with KDE 3.5 where the fonts were normal in some programs and way too small in others. In Firefox, the fonts of webpages were normal, but the fonts of the controls (like the web address, File, Edit, View, etc) were too small. The font used for YaST was barely readable.
The fix is pretty easy. Fire up the KDE control center, either by choosing "Configure Desktop" from the Gecko menu or running kcontrol from the terminal. Choose "Appearance & Themes" from the left hand menu and then choose "Fonts". Set "Force fonts DPI" to either 96 DPI or 120 DPI. On my Eee PC, I chose 120 DPI and now all of my fonts are nice and large and readable.