Real people have all sorts of proportions. For example, they might have unusually long or short legs for their size. But statues often have ‘ideal’ proportions. These ideas of ‘perfect’ human proportions began with Ancient Greek and Roman statues.
Artists usually measure figures in numbers of heads. The diagram from a textbook on anatomy for artists shows a figure 8 heads high. From the top, 2 heads down are the nipples. 3 heads down are the belly button and the elbows. 4 heads down (the mid-point in the figure) is the wrist and a line across the bottom. 6 heads down is the base of the knee-cap.