
I was experiencing this issue not only in KDE, but while using MythTV as well. Here is what I did to resolve it:
As user root from a terminal edit the /usr/pluto/bin/Start_X_Wrapper.sh script using vi or nano
sudo -i
nano /usr/pluto/bin/Start_X_Wrapper.sh
Look for the line :
xinit /usr/pluto/bin/XWM_Wrapper.sh "${Parms_Client[@]}" -- "${Parms_Server[@]}"
And add either -dpi 96 or -dpi 100 to the end of it like so
xinit /usr/pluto/bin/XWM_Wrapper.sh "${Parms_Client[@]}" -- "${Parms_Server[@]}" -dpi 96
And reboot the machine. This should make things quite better for you.
Hope it helps.
Seth