Great sized cage and good advice!
I have mine in a 48" x 30" x 20". I like to allow semi arboreal snake some head room, but there is certainly nothing wrong with 18" high. 18" tall is a very safe bet for optimal heating and holding humidity.
To the OP, when you say "red tail" do you actually mean a BCC? Depending of the sex and the local of the animal, you could potentially be facing a 10+ foot 80 pound animal. It's rare, but a possibility.
These cages have a 48" footprint. The top is 48" x 24" x 14"(wish it was 18" tall) and the bottom cage is 48" x 30" x 20" (perfect!)
Pick your boa and cage with some forethought.
If you do your homework it will be very rewarding.