We have a breeding pair of red-tails that live together. While both are under 5' now they are obviously going to grow up! We're building a custom enclosure 8'x3'x3'. The female's momma is just over 10' and still growing, so we figure that the enclosure should be at least 8' long when she's an adult.
Also, at 3' high it's a good height to put some smaller enclosures for things like ball pythons on top, if you build the boa enclosure sturdy enough to handle the extra weight.