i don't have literature to back this up but i thought it was dependent on climate. if it was hot, they tended to be taller and thinner as that allows a greater surface area for sweating and heat regulation. if it was cold, they tended to be short and stocky to conserve heat and stay warm. i would guess a medium height for those in milder climates. i don't know, makes sense to me and i know i've heard someone say this before, just don't know where.