Go into the KDE menu and select Settings
Go to the Workspace section and look in the subsection labeled Startup and Shutdown
Go to the tab labeled Background Services. Look in the list that is located to the right and look for one that is titled KScreen 2
Locate the pause button for this service which is located to the right side of the screen right across from the KScreen 2 label.
Click on the pause button then uncheck the box that is located to the left of the label KScreen 2, then click on the apply button at the bottom of that menu on the right side.
Then go to the Display tab and then select the screen resolution that you desire and then click on the apply button at the bottom on the right side of the screen.
Make sure that you save the settings because of the window management of KDE and the KScreen 2 service that is running in the background. Unless you are installing on bare metal the system cannot detect a virtual screen size in order to auto select a different resolution.