That's a good question! I've often wondered that too!
For me, I wear a 7.5 shoe. The biggest I'll buy is size 8 and that's only depending on the shoe itself. Sometimes, with the very high heel, the angle of the shoe will push my foot down towards the toe, and I'll get the 1/2 size larger to give me some room when I walk.
But I try to make sure there's no "gap" at the back of my ankle/heel area.
I've always had a thing for high heels. And I do wear them out, not just for photos. But you won't see me doing the weekly grocery shopping in a pair of 6" platforms
I'd have to say, my favorite brand is Pleaser