If it's a pure Hog Island, and you don't overfeed it, then a male could live it's whole life in CB70 and a female would be fine in it until she was fully grown. A female might max out around 5ft and she could move into a 3' x 2' cage, or maybe a 4' x 2' cage max.
Where are you planning to purchase one from? I have corns and ball pythons, and just purchased my first Hog Island from Vin Russo and she is by far the most personable snake I have. She's a 2012 and I currently have her in a 14qt tub and it's plenty roomy for her.
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Also, I've held a few 2 to 3 year old Hogs and they were about the size of a 1-2 year old ball python and would have had plenty of room in a 28qt at that size. I've heard it takes them about 4 years or so to reach sexual maturity and max out.
I am planning on buying from either vin Russo or rio bravo reptiles. That is good to hear that your boa has a great personality. I was hoping for something with a bit more personality than my "pet hides", even if I do love my BPs they can be quite boring at times.