A 15 x 8 to a 17 x 8 or 18 x 6 depending on it's state of tune Captain. I have one Echo 23.6cc that is stock except for running Gas/Glow, free breathing muffler, and an 11 mm carb. I run an old wide blade Top Flite 18 x 6 on that one. It turns that prop at 7500 and hauls the big Great Planes Fokker Triplane around really nice.

Then I have a fully hopped up Echo 23.6 on a 72" span Katana. I run an APC 16 x 8 on that. Turns over 9000 and flies well too. Depends on the plane and what you want, as you know.

AV8TOR