It's more about inside leg and body length than frame size in inches, i'm 6ft 2" but i've only got a 33" inside leg at best but a long body, i can get away with a 17" with a long stem.
Just swapped my 16" MsIsle which was too short and would of needed all of a 400mm post out the top to pedal correctly for an 18" 456 carbon which I've manage to feel right at home on.