If your pet shop has a REAL Hogg Island Boa, it's probabably not going to be much larger than a 2000-2500 gram adult female ball python. They grow on average from 4-6 feet with not many of them ever reaching the 6 feet length. Males will be closer to 4-5 feet long.
Male Dumeril boas usually stay around 5 feet long too. However the Hogg Island boa is a much more docile boa. Some Dumerils can be agressive, but I've never seen a mean Hogg Island boa.
I've bred both and if you're looking for a great pet snake, the Hogg Island is hard to beat. The Dumeril is a great looking snake, particularly when bred for coloration. Good genes will make a beautiful snake.
Good Luck!
Jim Smith