For fullscreen gaming with Wine and Xquartz, I'd suggest looking into a 3rd party tool I make called Wineskin (http://wineskin.doh123.com/)... gaming is the main focus, which includes fullscreen.... Things can be launched in fullscreen mode and global resolution changing can occur... I still have some issues to work out, its far from perfect.. but the goal is to let you make a Mac .app of any program that runs looking like as close as I can get it to a native app as possible. Gives you all GUI functionality... and many Wine versions, no needing to build them yourself. I always build the latest Wine releases for it, usually the same day they are out... and it has its own version of Xquartz built in, it doesn't use your X11.app. 800x300 is an odd size, your Mac wont support. When it tries to switch to 800x300, Wineskin will just switch it to the closest it can get, which would be 800x600, and the game would be on the top 300 pixels of that.
