I'm considering purchasing a baby boa. The snake in question is a male, and is advertised as a "common boa". Question #1: Does this label indicate a particular species of RTB, or should I ask the seller for more details?

Question #2: I was told this snake will require a 4-6 foot cage as an adult. I currently own a Boaphile cage for my ball python, and if I purchase this boa I will probably order another for the new snake. WWYD: Is it probable that a 4 foot cage will be sufficient for the life of the snake? Or would it be better to order a cage with an expandable end option (and possibly start with a 3 foot cage for now)? How long would a 3 foot cage house an 08 boa before I would need to expand?

Thanks for your input!