There are two general guidelines that I have always heard people use:
1) 1 square foot of floor space per 1 foot of snake length. That would mean that a 4' X 2' enclosure could hold up to an 8 foot snake.

2) The length + width of the enclosure should be greater than the length of the snake, in which case, the same 4' X 2' enclosure would only be suitable for a snake up to 6 feet.

Since my boas are smaller species I haven't had to deal with caging for a large boa in the 9'-11' range, but I would say, for a snake that big, the smallest enclosure that you could get away with would be something like a 6' X 2' and something like an 8' X 3' would probably be your best bet...