It really depend on the picture type. There is pictures that are used mostly for blogs or web usage.

As example this is 2 exclusive Plus images. This one have sold over 70% of the times as XSmall and Small on Istock
 


XSmall   Regular   2.30
Large   Regular   8.80
Medium   Regular   6.30
XSmall   Regular   2.20
XSmall   Regular   2.20
Small   Regular   4.60
XSmall   Regular   2.20
XSmall   Regular   1.75
Small   Regular   2.40
XSmall   Regular   1.90
Small   Regular   3.70
Small   Regular   3.80
XSmall   Regular   1.50
XSmall   Regular   0.76
XSmall   Regular   0.70
XSmall   Regular   0.48
Medium   Regular   3.70
XLarge   Regular   7.60
Large   Regular   4.95
XLarge   Regular   5.80
_____________________________________

This one have sold often for larger sizes



Large   Regular   7.00
Large   Regular   9.00
Medium   Regular   6.75
Medium   Regular   6.60
Medium   Regular   6.60
XSmall   Regular   2.25
Small   Regular   4.40
Medium   Regular   5.85
Medium   Regular   5.85
Small   Regular   4.40
 XSmall   Regular   1.45
Medium   Regular   5.85
Medium   Regular   4.05
XSmall      Regular   1.90
XXXLarge   Regular   11.55
XXXLarge   Regular   12.25
Small   Regular   3.50
Large   Regular   3.60
XSmall   Regular   0.59
Large   Regular   3.60

It depends on the subject but I'd say the majority of sales for most people are XS, S, and M with anything above that only happening a smaller percentage of the time.