You can also try to add this lines to Planeshift.cfg (instead of using pssetup.exe) using the Notepad.
Video.ScreenWidth = 1024
Video.ScreenHeight = 768
Video.FullScreen = true
With a dark box thingy between the commands (you will see what I mean when you open the file with notepad).
Obviously the numbers 1024 and 768 represent the resolution @ which you want to play so replace em for your own. And Video.Fullscreen = true makes it fullscreen (doh).
It worked for me, although it\'s a bit bugged now... I think I need to restart my pc

Good luck,
-Razu Kha\'el-