For most key brands, though, the possible pin sizes are finite and discrete, so you just have to estimate the height of each pin from the picture and round to the nearest one. It's not that hard.

Why wouldn't you have prior knowledge? You presumably know where it goes - just look at the keyhole. Or if not, a single identifiable object in the photo gives you a lot of scale information. I doubt they're as concerned about white-background newspaper-worthy shots like the NY keys, and more about shots at a distance (keys on a table, for instance).

edit: http://www.jacobsschool.ucsd.edu/news/news_releases/release.... for an example