MTB's and road bikes are built differently with MTB's having a longer top tube as the norm for frame size. I ride a 51cm Road bike that equates to 20. A 20" MTB is a lot bigger than I could handle. I ride a 15" one but in different manufacturers could ride an 18" and it would fit perfectly.
How a Mixte frame fits in I could not say but womans road frames do have a shorter top tube for Frame size and I believe the MTB's are the same.
__________________
How long was I in the army? Five foot seven.
Spike Milligan