Ball pythons really don't exceed 5', ever. And the adult Hogg Island boa they've got at the pet store down the street is actually -quite- large compared to my adult female balls.

If you really must have a red tail, Mexican/Sonoran boas are considered "dwarfs", though it's hard to find ones that aren't mixed with central/south american species.

Have you looked at rosy and rainbow boas? Or tree boas and pythons? There are also a couple australia species of python that are even smaller than balls--children's/spotted pythons, for starters. Though I've heard balls have a sweeter temperament.