My understanding of helicopters is that, despite theoretically increasing efficiency, longer blades are worse in practice than short ones in every respect (except, perhaps, cost):
Longer blades are more prone to vibration and resonance dynamics, if for no reason other than that their rigidity is reduced by length, holding all else constant.
Longer blades provide less operating clearance.
Longer blades not only stall before shorter blades (retreating), but also hit the Critical Mach number at lower air speeds (advancing).
Fewer blades require higher rotational speeds for a given "rotor disk" area, which is inefficient from not only an aerodynamics perspective but also, presumably, a transmission-and-control perspective.
But all helicopters have few blades of significant length. So: What are the advantages of longer blades, or disadvantages of shorter blades, that lead to the rotor designs in common use?
I would imagine that in the limit it would be optimal to run something more like a ducted turbine segment, where the swept area is virtually covered in blade surface (i.e., maximizing the number of blades over the swept disc).
Noting that in practice blade size seems somewhat proportional to the body size of the aircraft, I'll deduce that in practice the bulk of the downforce needs to be generated some minimal distance away from the aircraft's body, though with some attention to the body's vertical aerodynamic aspect it seems like that could be reduced. I guess tilt-rotors could support this hypothesis since their rotors are positioned at a distance from the body and they use smaller rotors, although it seems like they still use blades as large as possible given the wing size and body configuration.