Little person is probably correct. Looking at this realistically you may ask what is PC for a really fat or tall person? People are people. If you know them just refer by their given names. Refrain from size description as we do (usually) in respect to skin color or ethnicity.
If you want to be awfully PC then Boo153 hit it with vertically challenged. If you want to win friends, follow Rathkeale's answer. But if you want to be absolutely horrible you could do worse than take a few pointers from this song of Randy Newman's. It's called Short People. http://www.youtube.com/watch?v=1NvgLkuEtkA