A good friend of mine keeps his adult females in 4 x2 cages and they do just fine. I like to over size my cages for the fun of it, but I also have a small collection. Boas truly are not very active. Mine is a yearling and in a 4 x 2 cage, he literally sits in one corner all day, at night he MIGHT come out to look at his water dish. I just like to make sure that the dish is big enough for him to soak in, he likes to soak. I guess just get to know your snake and make the decision from there