How do I change my display settings if I can't see my screen?
Solution 1:
Try to to go /Library/Preferences/ and search for com.apple.windowserver.plist. It's the file that gets updated when you modify the display in System Preferences (I just tried).
According to this Apple KB, it's safe to delete it. So move it on your desktop and try to reboot.
Solution 2:
From the command line, cscreen lets you adjust monitor resolution.