Depends upon the plane and what you want it to do. I have mine in a H-9 Taylorcraft and I am using a 20X6 MA. It flies very realistically, does great acrobatics, and lands terrific.

You can goto 8 to 10 inches of pitch and you can have more speed if that is what you want. Likely you will need to go to an 18 inch diameter for those pitch choices.

Buy different props and experiment til you get what you want. Bye the way, the engine runs flawlessly. I did solder over the hole on the butterly to get better fuel draw while choking. The mod took about 15 seconds.