RE: How do I choose prop size?
More pitch typically equals more speed.
The engine has a certain amount of power so you have to use different combinations of diameter and pitch to keep the engine in the right RPM range.
These are made up numbers but hopefully illustrate what's going on
Prop Max RPM Thrust Speed
12x10 10,000 4 pounds 100 mph
13x8 10,000 5 pounds 80 mph
14x6 10,000 8 pounds 60 mph
15x4 10,000 10 pounds 40 mph
REPEAT, THESE ARE MADE UP NUMBERS.