What size saddle would a 13.2hh fell pony need?

Saddle size depends completely on the pony and it is difficult to say without measuring. The best way to make sure your saddle fits is to have it professionally fitted. Ideally, start out with a "pony" saddle, and your seat size (in English saddles measured from the point of your hip to your knee, in inches, and measured on the saddle from pommel to cantle).