Due to lack of good information on the net and because this is asked alot by people interested in Boas I thought I would take some time to explain the factors that make up the size difference in boas.
How big will your boa get? On average Boa constrictors will be between 1.7 - 2.2 meters or 5-7 feet. This is determined by three very important factors that affect breeding and size.
Factor one: Sub species.
On average you can expect True red tails (Boa constrictor constrictor) to be larger than their smaller Central American cousins Boa constrictor imperator who most in all grow to only 1.5 meters or 5 feet in length.
Factor two: Locality.
Locality plays a large role in the Size and Coloration of Boas. While the Central American Boa constrictor imperator are normally smaller in size the Colombian Boa constrictor imperator can grow to lengths even larger than any true red tail. In true red tail boas B. c. constrictor the Peruvian locality is normally larger growing.
The last factor in the size of Boas is Genetic disposition. There is one line of small growing Surnam red tails that only grow to a length of about 1.7 meters or 5.5 feet that have been line bred from smaller boas. The size of the breeding line determines how large the boa could grow.
I hope this short explanation is helpful to some.