It shouldn't be much of a problem
Just go into the "Configure Computer" from your K menu and go to the hardware tab and click on configure Monitor or Configure graphics.........im at school right now in study hall so I do not have Mandriva running but it's one of those tabs on the left side and then you should be able to choose what resolution you want it to allow and then restart and right click on your desktop like you did before and change it from there..........if you have any more problems, school gets out at 2:30 today so I should be back on the forums.