Definitely need to experiment. It depends on what kind of flying you do. I've been using an 11x5 prop on the 46AX on high wing planes. Next plane is a Dolphin (copy of the Four Star 40) and I'll be using a 12x5 prop to start. This will give me a little more pull at low end, while sacrificing a touch on the high end. Generally when you go up a size in diameter, you drop a number in pitch. The lowest pitch OS recommends for an 11 inch prop though is 11x6, so it would make sense that a 12x5 would create about the same amount of resistance on the engine.

Manual says for sport flying: 10.5x6, 11x6-8, 12x6-7/

These are just reference points, though. If you want some real top speed, try a 10x7. If you want to do more 3D stuff, try a 12x4. Something in the middle would be an 11x6 or a 12x5.