figuring out the size is always a good start, and for the most part that is figured by the needs of the person and of the animal, placement, also cost commonly plays a big factor. many people will go as big as they can giving the area they have to work with. you sound like you have plenty of room to work with therefore next look at the animals needs... well IMO BRBs can do just fine in a 2' cube or 3'L x 2'W x 1'H to as much as a 10' x 10' room... some people will say dont go to big, mainly due to common sense the bigger the enclosure the harder to maintain husbandry.... i say if if you have the space and mony is a nonissuse, plus can maintain proper heat and humidity, keep it in a 50' by 100' if you want.
i cant say for sure because i dont know for sure, but i have always been under the impression that BRBs are somewhat climers. i would bet that if you used ledges or shelves up higher with warm moist hides they would get used use.
the other thing that will help determnine size is are you gonna buy or build? what kinda of look are you going for, do you want a simple plastic box or where you wanting something that looks a little more fancy?
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-Stephen-
0.1 soon to be wifey (Babe)
1.0 rotwiler/chow (Boomer)
1.0 norm corn (Jake)
1.0 col redtail boa (Switch, formally known as Dixie)
0.1 ball python (Bella)
1.1 Brazilian Rainbow Boa (Torpaz & Saphira)