"True" red tail boas are the BCC boas such as Peruvians, Surinames, Guyanas, etc
Colombian BCI boas are commonly called "red tailed boas" though it's not totally correct IMO. I don't call my Colombians "red tailed boas".
Bet on at least a 4' cage for adults BCI boas, and I recommend 6' for adult females and large males. You can look into dwarf boas but sometimes they can get just as large as Colombians, and if they stay on the smaller side they might not have the girth you're looking for.