Part of the answer on single and twin rotors: Rotor tip velocity is limited by Mach on the advancing blade in forward flight, so as your rotor gets bigger its rpm is perforce less. More power + lower rpm = lots of torque, which combined with a wider ratio in the transmission is mechanically challenging - the weight of the transmission and shaft increases more than proportionally with the power.
Have you seen the transmission on an Mi-26? I remember when they brought the beast to Paris and opened up the cowls. It's like a Volkswagen made from solid titanium. More torque also means more power to the tail rotor - even relative to the size of the helo, the tail rotor is huge.
You can squeeze the diameter down as much as possible, but then you get jet-blast downwash.
The bigger you get, the bigger the advantages of multiple rotors - which is one reason why the -47 has lasted so long.