You don't need to worry about keeping things too small for boas. They are not as insecure as balls are. I start my tiny babies in 32qt tubs. When they are about 8 months old or so, they go into 41qt tubs. Once they are roughly 1 1/2 years old, they go in their adult enclosures. I just give them lots of hides and they do great.

** It's important to note that not all boas grow at the same rate. I go more by size than age to determine who gets what size tub. I have 2 2011's that are in adult enclosures and 3 of the same age that are still very comfortable in 41qt tubs.