It depends on your sleeping habits and temperament, and the temperatures at which you'll be using the quilt.
Width: I'm 6-4, broad-shouldered, and mildly claustrophobic. The only complaint I've had with my Nunatak quilts (large size) is too tight across the shoulders and chest. That may be due to my side sleeping and regular tossing and turning, or just longtime use of semi-rectangular sleeping bags. If I were a quiet sleeper I think the width would be OK.
Length: Definitely a regular length will suffice. If it gets cold, layer up with a down jacket and hat. That's the point of a quilt.
You might check out the quilt reviews (some of them mine) at BackpackGearTest.org to get a better idea of what you want.