Suspect there are too many factors in this one to give a solid useful answer.
"It depends" is probably the real answer. Depends on the customer's needs, the subject, the price the artist set.
My print sales today are typically on either end of the spectrum, customers buying the largest size I offer and some the smallest. Some pick in the middle sizes, probably based on a space they have to fill, but I can tell most are just going for either end of the offered sizes. Years ago when my prices were much cheaper, they nearly always went for the largest...
Anything you would likely get from FAA would at best be an average that would only tell you what sells at a given size across a multitude of art styles, prices, etc. It wouldn't tell you what size will sell best in your style/subject. Or what effect marking up that size would have on your individual sales.
It's one of those statistics that could be sliced and diced so many ways. What size sells best in X style/subject? What size sells best when the artist mark-up is over x value or x percent of the base cost? What size sells best when the economy is up? What size sells best when it's down? Etc.