I have an RTB. She is about five and half feet long, and weighs about ten pounds. I keep her in a HUGE plastic tub (you can find them at Reptiletubs.com... or Wal-mart sells them during the holiday season).
One thing: my boa is not normal. She is small for her age and will always be small. Normal female boas can get up to eight feet long (sometimes longer) and they require a bigger cage than what I have mine in right now. You'd have to get at least a 6x2x2 cage. Males are a little smaller, and could probably remain in a huge tub for their lives.
Check out caresheets before you get any animal... and check out more than one. I'd find four or five, just so you can get a good range of information, and find our what's right and what's just made up.
Hope this helps.