GG reads the screen res automatically for test games.
You would have to set your screen res to 1280 x 720 before starting test game.
Hopefully this will be adjustable in the future.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit