If you have decided on a Boa I would recommend a female BCI if you would be alright with 6' - 8' as an adult or a male BCI if you would rather it only topped out around 4' - 6'. If you want one that tops out at only around 4' or so and you like dark snakes you should check out the Leopard Boas, which are dwarfs. They are gorgeous! A Leopard Boa/Motley pair is probably next on my list, because i'm really wanting to get into the Eclipse project. Either that or a pair of super dwarf Retics. I can't decide...
Have you considered a Sumatran, Blood or Borneo Python? A Woma Python? There are just sooo many awesome snakes that it's impossible to choose just one! lol If I wasn't getting into breeding and I just wanted a couple/few pet snakes a Boa would be a must have, though!