Ok. I'm looking for a new horse and i'm wondering if me(5'7) would be able to ride a 13.3hh (16 year old). I'm thinking it's a long shot but any tall rider with a small pony?
(I don't know how thick the horse is or breed.
I also used to own a 12hh welsh pony. I was obviously a few iches shorter though)
Depends on your build, the horse's build and what you want to do with the horse.
A sturdy 13.3 H animal is probably up to carrying your height without difficulty; the other dimensions by which a rider "fits" a horse are individual and dependent partially on discipline.
For instance if you ride in a discipline that requires you have your lower leg firmly on the horse; it's more difficult if your heel is well below the barrel, you want the horse to "take up your leg." In jumping disciplines, a rider with a long upper body on a short coupled horse is going to have trouble with balance and be more likely to weight the horse's front end while out of the saddle.
So the best thing you could do is post photos of you on the pony, and let us know what type of riding you plan to do.