A little bit of trivia:
I work with a guy who has crewed for the Rare Bear. It has a 13 foot 4 blade prop, a 3:1 gear reduction from a P-3 orion. The throttle is set to wide open and pitch is adjusted to limit rpm to 3200 rpm, above that and the tips of the prop go above mach which can be heard. The blades are 18" wide. This combination has produced speeds in excess of 500 mph.

Ground clearance and prop tip speed are two big considerations for full scale aircraft.