I would not got one just because you can get one for $60. If it comes in from an unreputable source, I would pass on the deal. A boa coming in with IBD can wipe out most other snakes that come near it. A 55 gallon will more than likely not hold a full grown boa. If you insist on a cage, a 75 or 90 gallon (i forgot how big they were) will be fine. Whichever one is 4x2. Isn't a 55 gallon something like 4x1.5?