Bit of a strange one, but CSS resets my resolution to 800x600 every time I close it. Can't work out why and the steam site has nothing on it. Any ideas? It runs the resolution I want fine but when I close it it's just gone.
I've had this problem before too. Add this to your launch options (right click game, properties, launch options) to force the resolution:
-w 1024 -h 768
Or whatever resolution you want.