4' x 2' x 2' (L x W x H) will be fine, though there is no need for the extra height. A 4' x 2' x 1' Will be easier to heat and maintain humidity in. This is what I use. I buy my cages from a company called animal plastics. Their T8 cage can house 1 BP or 2 if you buy the optional cage divider.
The cage on top in the picture below is a T8 (4' x 2' x 1') The cage below it is a T12 (4' x 2' x 2') Below that is an 8" stand for the cages.
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