Daniel:
"I think you'll find that momentum theory will tell you that larger is always better."
I totally agree, if you are talking about a hovering rotor.
If the helicopter only needs to hover, then the induced velocity can be 500 ft per second (through a minuscule rotor) or 0.5 ft per second (through a monstrous rotor), since the craft is not going anywhere.
If the craft is required to climb at, say, 50 ft per second, then the induced velocity must be at least 50 ft per second.
The same thing applies for forward flight and this is probably one reason why faster helicopters have a higher disk loading and a smaller rotors.
I speculate that this will also apply to propellers. If the intent is to have a fixed wing craft that will fly at 250 kts (422 fps) then the induced velocity through the propeller must be greater than 422 ft per sec. The designer will pick an optimal tip speed and an optimal pitch for this forward velocity. Then he will increase the size of the propeller until it is large enough to overcome the drag of the craft.
In the case of this thread's proprotor craft, the proprotors should be large for an efficient Figure of Merit in hover, but they will probably be quite a bit smaller when providing the desired forward velocity in cruise.
Dave