Both my horses are basically ponies. They stand within an inch of one another in height, both in the 14.1-14.2hh range. However, they're built pretty different. My QH is fairly broad and has a long back, so she can easily wear my 17" Toulouse jumping saddle. My Welsh/Arab mare is relatively narrow and a little shorter in the back, and I have to ride in a 16" Stubben youth dressage saddle on her. The length of the Toulouse is ok on her, but the flap seems to interfere with her shoulder. It also has to have a rear riser since she's much more uphill.
Depending on your pony's build, you might get away with 16.5-17", but 13.2hh is 3 inches smaller than the mare I need a youth saddle for. Youth saddles may be your best bet. Stubben makes some nice ones, and they wear well. Mine is at least 30 years old. It's a bit small for me in the seat, but I do appreciate the short flap since I'm only 5'2".
I guess part of my point is that it is not only back length at issue. Everything on a 13.2hh pony is smaller, so you need less flap, too. Do you have a saddle fitter in your area to help decide how big you can go?
Tug on anything at all and you'll find it connected to everything else in the universe.~John Muir