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