Thought these pictures might be a good size reference for you..

This is my 4'x2' 12" high RBI cage that I keep one of my small adult female RTBs in (She is pretty comparable to adult male RTBs I have seen before - she is 5'8" and 16lbs - Hard to tell since she is all curled up in the pictures though)







And my house is not as dirty as that room! This snake is in a kind of quarantine situation so she is in the unused half of my bathroom.

If you can afford the cage and space your snake will not mind a larger cage - but the 4' RBI cage will work for an RTB who isn't monstrous in size.