Old thread...but if anyone's still interested, a 12X4 APC works well for fun fly and 3D type flying, and a 10 X 7 for a sport type...
At least that's what I have been using with the Alpha engine (.455 )
I put an 11 X 5 on the engine when it was still on the XE-2 and that worked much beter IMO than the three blade 10 x 4 or what ever the pitch of that paddle prop is.