From what I can tell 6 feet is considered extraordinary - ready for the snake NBA. Or WNBA I should say since females are larger. I get the impression that 3 to 5 feed is more likely with around 4 feet as average.

I've read they grow about an inch per month until they are 3 years old. You might be able to estimate the final length by looking at the age and current size.

In any case, BP's are usually fat for their size.