According to the Dair Group, 28 percent of buyers purchase a size medium size, then extra large is next at 20 percent. The percentages are much lower for smaller and larger sizes. In total, it breaks down like this:
XS: 1 percent
S: 7 percent
M: 28 percent
L: 30 percent
XL: 20 percent
2XL: 12 percent
3XL: 2 percent