There is a problem with the boards we are fixing this.
Until then If you try playing in live premium not live simple, make co-ordinates so they show inside the board not outside, and use the F11 on your keypad, that should make the screen full size for you.
http://live.chess.com/live?v=2011072801#
My board size has shrunk down real small and I cannot change the size. I can change the style ok but the size settings will not take effect when I try and save the change.