Going by the fact that I am 6'0" and ride a large 19.5" mountain bike this sounds like it should be the ideal size for you. Just make sure the 20" is referring to the frame size and not to the wheel size!
it depends on your inside leg, if you put your seat to the height you
want it then sit on it and put your chainset so the crank arm is pointing
forward and put your foot on it,your knee should be on line with the
peddle axle