Charlie my first BCI came in a 55 gallon tank. He is 6.5-7ft long and as big around as a coffee mug. I did not feel that the 55 was at at big enough for him. He almost never moved around and he had rub injuries on his nose from the enclosure being to small and it was very hard to get a water tub big enough for him to soak in inside of it. My boyfriend and I built him a custom enclosure. However my 2nd Boa Jane is about 4-5ft long and I house her in one of those huge reinforced plastic christmas tree tubs. I bought it in Feb. at a discount. The only downfall on that one is she will eventually out grow it and it's lid is green with red latches. If you are going to use a tank I would go with a 75 at least for a full grown RTB. They are about the same length but they are a little wider which will help out a lot.
If you already have a 55 it will work for you for now. It should take your little guy a few years to out grow it.
I hope you enjoy him. RTB are one of my fav.