There are different factors involved.
Breeding, feeding frequency, prey size, external/environmental factors also play a role.
If you caught April's post you may have noticed she had a 28 year old boa. Not only is that a wonderful, long life, it is a sign of an excellent keeper. I don't see many folks with truly old boas.
One quote I really like by Gus Rentfro goes like this. "In nature, the largest boas are the oldest boas."
His point was people can power feed a boa up to a certain large size and it will ultimately die early and never reach its true size potential but the successful boas in the wild eat infrequently, grow slowly and don't die early due to over eating. Those are the ones that end up big, depending on the subspecies.
This is a 7 year old, male locality BCI. A Barranquilla, Colombian that was bred by Gus Rentfro.
He is 7 feet long or just under. He's a good sized boa, but not huge.
He may be larger than the average male and that may be due to the fact he doesn't have any female boas to cue off of in the house.
I feed him monthly, or a little more frequently in the summers.
He does not eat from about mid to late October until late March or early April.
If you want to get an idea of what size your boa will end up, look at both parents and estimate from there.
Never be in a rush to have a big boa, they get there when they get there.