I'm on a Surface Pro 3 and when I click fullscreen, it blows up too big and I only see about a quarter of the screen. I had to delete the game and reinstall to play.

I have the same issue on my Surface Book (and I think it unfortunately happens on most Windows machines with HiDPI displays). Still need to figure out how to fix that.  One thing you can do though is hit F11 to toggle the game back to windowed mode (should work from the Main Menu and during gameplay).

I suspect this is an issue with the wrapper I'm using for the Windows build (packr).  If I launch the game directly from the .jar file (MewnBase/game/desktop-1.0.jar) - the game launches small, but centered on my display, and fullscreen seems to work properly.