I see, sorry.
Usually you might be able to do that in your KDE Contol Center. In the peripherals section you'll find a display menu. But this is, as far as I know the same applet like the "Size and Orientation" one - so if the changes you make there have no effect, I think that your problem is elsewhere.
In this menu, do you have several screen-sizes?
Eventually different refresh rates (with some cards/monitors there'll be just one)?
Do they match with the card/monitor specs?
If yes, changes you make there do not take effect once you click the apply button?
If they don't, do you have any error messages from your system?
Messages could also appear in the file /var/log/messages or in /var/log/xdm.log (or kdm.log, and perhaps in one of the Xorg.x.log).