It depends on what locality of boa we are talking about. If its a true red tail such as a surinam(sp?) a 6'x2' cage should suffice for an adult. If it is something like a columbian red tail boa a smaller cage of around 4'x2' should work for you. Hope this helps a little.
The biggest you can go within reason is better. An 8' boa could live in a 4' by 2' cage, she may be crampt a bit. I would say for anything in the 8-10'+ range, a 6' by 3' would be a good idea.
~Jake~
Too many boas to list and a few balls as well