I would go with a Common boa(BCI) or a Super Dwarf Reticulated. My female boa is about 3 years old and is 8ft and 25lbs. She's a handful of muscle and when she wants to go somewhere, she'll go.

Super Dwarf retics are nice because they still have the Retic personality, but not the size. They get maybe 5-8ft(right?) and are long and lanky, not as girthy as a boa.

So.. You don't necessarily HAVE to get a full grown Afrock, Burmese, Reticulated, etc. There are tons of other species of larger snakes, that don't get AS large.

Look in to Morelia, Liasis species. They get "robust" but not exceedingly large where they would take up half of a bedroom