What is the maximum current rating of the GWS motor? Right now, you have about 45 watts of power (2*3.7*6). 42 watts/pound (45/ (17/16)) is low by most standards. But if you can increase the current load to 10 amps, then you would have 70 watts/pound, which might be enough to fly the model reasonably well.
Before you increase the propeller size, make sure the battery, speed control, and motor can handle the higher current load. You can find this out by looking at the specs.
Off the top of my head, I do not know how much thrust a Cox engine produces. But it should not be too hard to figure out how fast it turns that 6x3 prop. Then using my calculator you can get an estimate of how much thrust you had.
