I am 5'11", slim, 32" Inseam and ride a 19.5" frame, tried a 18" frame and felt like a clown on a circus tricycle..

I tried a 21" frame and It was a bit hard to move beneath me and super tight switch backs were wobbly. I felt too stretched over the top to make super slow maneuvers work..

I know a 6'4" long leg rider on a 21" bike, he tried mine,, no go, no way, he could ride a bigger frame I think..