I had that same problem myself and here's how I fixed it.
first a bit of background:
I use a laptop with a screen that support 800x600. Whenever I ran gnome, screen fonts were normal, but when I ran KDE screen fonts were HUGE and made the entire thing unuseable.
I looked in /etc/X11/xorg.cong and I removed the references to the 100 dpi fonts in the files section.
After doing this I was able to run KDE without having the fonts being huge
Here's what I made the font list look like:
I hope this is of some help to you