lol if you dont want a big boa, never put it on rabbits.
I have seen small 6 - 7ft females, couple years old like 4 - 5 years.. then once they are started on rabbits within like a year or two they are pushing 8 - 10 foot.
And, I cant wait for Powder to become a giant.. she is already insanely big for her age (3ft 4 inches, about 500 - 550 grams age... 9 months LOL)
Just my 2 cents. I plan on feeding Powder rabbits as soon as she can take them! They are better prey items then rats.. I dont know why but they seem to have more.. nutrition?
Anyways.. as I said, just my 2 cents
edit:
Oops, to answer the question simpler... females average 6 - 10 foot. Genetics, precise heating, feeding and other things like that will decide how big your boa is