Size of enclosure can depend on which Boa species you are most interested in. Generally, a 2' x 4' x 1' (d x l x h) cage will comfortably house an adult Boa Constrictor constrictor (true red-tailed boa) / Boa Constrictor imperator ("Columbian" red tailed boa).

Humidity: 50-60%, higher when in shed

Temps: 78-80 cool side 88-90 warm side/basking area

I think the temperment of your boa will really depend on the snake itself. I've known some crazy boas and also some very calm boas. It's one thing for a 4' ball python to strike at you or tag you, it's another thing for a 7' boa constrictor to not want you to handle it. Sometimes a snake like that just makes it tons of work to do some simple cleaning, etc... That being said, if you're planning on buying a large boa you may want to ask about its temperment first. Boas are very cool snakes, and very strong as well!