Open /etc/X11/xorg.conf file with the text editor with root privileges or run from the terminal;
Code:
sudo nano /etc/X11/xorg.conf
then look for the sections like Monitor or Display.Try to set things there and when you're done save the file and reboot.I think there is an option in the KDE Control center to set default screen resolution,search a little,use Google,use LQ search forums because your problem has been discussed many times before.