Herb,

All depends on the weight and drag of your airplane. Sometimes you will get more speed with a larger prop of same or less pitch.

Before switching to a Jett FIRE 50 I put quite a few flights on my Diamond Dust with an OS 46FX. Keep in mind it is only a 34" wingspan delta that weighs under 3 lbs but in my case I always went fastest with an APC 9X8 sport prop. I tried an 8X8 and turned more rpm with the same pitch but didn't go any faster because I didn't have enough thrust.

You will just have to experiment depending on the airplane will be flying but I would say with that engine 9 inch props would be a good starting point with smaller airplanes. For bigger planes you might have to take whatever you can get with a 10 inch prop.