I would personally ( as i always do ) Suggest a Caulker Cay Boa

They are small *Lighter than a BP, and around males lengths*, ( i can post pics of my ADULT female) Very tame ( babies are nippy, but calm down quick) and absolutely simple to care for. I keep mine just like ALL of my other snakes.

You can also look into Brazilian Rainbow Boas ( who need higher humidity, but that is NOT hard to achieve if you do your research, and are good with your husbandry) As well as Sabogae Boas ( Very Expensive, but very small, simple care needs)

Tarahumara boas are also small, but on the pricy side of things.

Check out Gus's website www.riobravoreptiles.com Guy knows his stuff, and has pages dedicated to many different kinds of boas, with lots of information regarding that species!

There are lots, that i can recommend. You just have to do your research on the species you end up deciding to get.