This whole concept that there is some mythical diameter above or below- which the efficiency of the prop drops-off is back in 20s or 30s technology. Try this: Tom Aberle at Reno in 2003 did 220 mph qualifying with a two-blade. In 2004 he did 240 with a 58" three blade at 250 rpm less than the two-blade of the previous year. In 2009 he did 252 at the same rpm as the two-blade with a 56" four-blade. I designed a 6-blade, 50" prop for a friend with a Blanton V-6 to turn 4200 rpm without the 75 lb PSRU. It turns 3200 rpm static and should come close to the design 4200 rpm. I'm now in the process of designing an 8-blade prop that may give over 7000 fpm.
It's all a matter of keeping the tip Mach reasonable, say 0.85, designing the planform to keep the critical Mach over the blade less than 0.75, and having enough blades to give the required mass-flow to get the efficiency. I've also designed a two-blade prop that turns 7250 rpm, and a three-blade prop that turns 9000 rpm, both for UAVs. Believe very little of what people say about prop diameters, rpm, and number of blades. And contrary to what some may say, prop design is not a black art practiced by the light of a full moon with various incantations and the tongues of newts. True prop design is very scientific and predictable.