Hi,
I'm no expert, but your 6 foot estimate seems about right. Like most boas and pythons, females usually grow larger than males. Remember, BRB's are very slender, so a 6 foot BRB is a totally different matter than say, a 6 foot blood python.
As far as temperament goes, a scan of previous forum posts would probably tell you that most BRB's grow up to have docile temperaments. Some books I have read even go so far as to recommend them as a good "pet" snake. Most hatchlings go through a nippy phase, but are easily tamed. Personally, my BRB hasn't offered to strike me and is very sweet, but it is obvious in her body language that she is not quite as easygoing as, for instance, a ball python. She is much more easily startled, and quick to assume the defensive "S". That's not to say that she's a handful, because she really is very docile, especially for a hatchling. She just requires more careful movements and gentle hands than some of the easier beginner snakes.
Another snake with similar size and beautiful adult coloration is the jungle carpet python. It is my understanding that carpets are much more prone to be nippy as babies than BRB's, but they also calm down with age. A male would be a better choice if the 6 foot range is what you're after, as female jungle carpets readily reach 7 feet or more.
Interestingly, jungles and BRB's are the two snakes I eventually narrowed my list down to, and I am also a first-time snake keeper. I ultimately chose the BRB over the jungle because I found them to be easier to handle and manage, and also of course because of their gorgeous iridescence. I don't regret my choice at all!
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My Growing Zoo:
1.0 Husband (Byron) ^_^
0.1 black cat (Shade)
0.1 Brazilian Rainbow Boa (Zia)
1.2 Cockatoo cichlids (A. cacatuoides yellow-gold)
1.1 WC Cockatoo cichlids (A. cacatuoides blue Peru)
3.3 Pygmy corydoras (C. pygmaeus)
2.0 Endlers' Livebearers (P. sp. Endlers)