Here is the general prop sizes in common use:

Most models fly at 60 - 90 mph wide open. Most glow 2 cycle engines are happy turning around 12,000 rpm range give or take. And so the 6 pitch is the most common for this combination.

A glow 4 cycle revs up a few thousand less, so 7 and 8 pitch is common for those. Larger gas motors turn even slower, typically 6,000 to 7,000 rpm, so 8, 10, and 12 pitch are common.

Then up or down a pitch or 2 to optimize for pull verses speed.

This all is for 2 blade props. Up the pitch one or 2 for 3 and 4 blades.

For model airplanes the diameter (the loading) pretty much follows the displacement. Glow 2 cycle .40s about 10 or 11 inch diameter, .60s about 11 or 12 inch, and so on....

And remember the pitch number is more of a manufacturer’s reference than an exact performance number. Blade design has a huge effect on prop performance. There are wide blades and skinny blades and sharp blades and blunt blades..... Your favorite prop size will often vary from one brand of prop to the next.

Multiflyer