Prop size equates to the gears in your car transmission. Lower gears give you more pulling power and less speed but use higher engine RPM. Higher gears give you more speed, but less pulling power at a lower engine RPM.

Same as props. The less pitch the prop has, the more power you will have for a given engine RPM but at a less speed.

A 6 inch pitch prop turning at 10,000 RPM will move 60,000 inches in one minute. 60,000 inches in one minute works out to about 56 miles per hour. The same engine turning a 12 inch prop at 10,000 RPM will go about 112 miles per hour. (all things considered, and in ideal conditions.. the plane design may not allow this speed, but this is possible, in theory). But the engine turning 10,000 RPM and moving that 60,000 inches in one minute, as with the 6 inch prop, will have much more pulling power than the same engine trying to turn a prop with twice the pitch. Same as the gears in a car or truck transmission. Cars that are climbing a steep hill will probably switch to a lower gear to get more power to the wheels in order to maintain the same speed. The engine increases in RPM.

CGr.