There's a variable pitch model using 450 sized parts and 2x600 motors around here somewhere. It's pretty large and impressive! There's also a blog post on a 1:1 quad if you go a few pages back. And, of course you've seen the smaller-than palm-sized models some of the universities have "on hand". The range is quite diverse, I think!
I've been playing around with the idea of 3 meter by 3 meter quad using 2 ft tail rotor blades a custom head assembly with variable pitch and variable conning angle shaft driven with clutches and an 80 hp motorcycle powerplant... Awww nothing like a quad that conceptually u could fly in if u were crazy enough lmao it would be a 200lber without payload
One large prop is more efficient than n smaller props; The advantages of a quad are the simplicity and efficiency of direct drive motors juxtaposed against the complexity of a cyclical etc. These advantages are presumably lost as the cost and size of the vehicle goes up. Your fixed pitch quad rotor of any size will crash, and with it whatever you carry; an adj. pitch rotor on the other hand allows for a dead stick landing, so your odds go up.
I suspect someone will build an n rotor with sufficient props to lift a human (~50-70)? and such may be safely redundant, but arguably a heli would be more effecient. (however, might it be oddly quiet - having a high frequency and lacking the main source of whoop-whoop (the tail crossing)?