I ran the APC 20.5 X 14 on all three I built and had the throttle governed back to make it easier to manage the excess power. That was with the Hacker C50-14XL on 5000mah 20C batteries. I tried the APC 21 X 14 and didn't like it, but don't remember why. This year I switched to an APC 21 X 13.5 to slow things down a little more. The APC 20.5 X 14 was a great prop but could get a little zoomy if you weren't careful with the throttle stick.
Verne
ORIGINAL: gaRCfield
Thanks Woodie. I was thinking mostly what's the best prop for the plane (do most people like this plane with a little more speed?) of course different motors run at different RPM so it is a system. But, it sounds like something like a 21x14 is a good place to start.