There are many factors that will determine appropriate cage size: size of snake, gender, activity patterns, opportunities for exercise outside of its cage, girth, etc. If you have a female, there is a good chance she will outgrow that cage in a relatively short amount of time (if I was making a cage, I would go with at least a 6' x 3' or 4' deep cage). My large females live in either 8' x 3' cages or 12' x 4' cages. If you have a male, there's a chance that the cage you are building will be adequate on a permanent basis.
Rob Carmichael, Curator
The Wildlife Discovery Center at Elawa Farm
City of Lake Forest Parks & Recreation (IL)