a 4' by 2' is all a rtb needs but going above and beyond won't hurt either. I have boas 2-4 years old that are still in tubs and have plenty of room. I use my cb-70 rack for boas until they get too big then they go in 4x2' boaphiles. I plan to eventually have 6' by 3' cages for bigger females but thats the biggest I will ever go in the forseeable future except for my first and favorite PET boa.