Standard door height is 6'-8". Minimum ceiling height is usually 7'-0". Some commercial doors are 7'-0" or even 8'-0", so personally I wouldn't go any lower than 7'. There are more and more 6'-8" people. Most of this world is sized for the 5'-2" female to the 6'3" male. Anything beyond that is optional. That's my story and I'm sticking to it.