I had this same problem with KDE CE Mint Darnya when installing the nVidia proprietary drivers that I am now having problems with in KDE Elyssa. When as root I let the necessary drivers be downloaded and installed for my geforce 8500 card, upon reboot, the log-in screen is in huge layout size, and fuzzy small icons. Logging in the icons and board layout is huge, and when I go to Monitor & Display, I see the resolution is locked at 800x600, and the slider bar won't budge. The only reason I wanted to try out this nVidia proprietary driver in KDE Elyssa, is that I noticed when I installed it when using Mint Gnome Elyssa, that the slight flicker upon logging in went away, so I thought I'd try it out and see as I have this flicker in KDE Elyssa. Right now, in Monitor & Display, it says nVidia GForce 8 series.
I notice some others have this problem, but I don't understand the work around for this. Should I just ignore downloading the proprietary driver? This driver also works with another distro I use that has the KDE desktop, so I'm not sure what to do.