... I see threads frequently where people are asking that question.. "How big should my boa be"? Usually they get good responses and advice.. In my opinion, however, there is no real answer! Look at these photos from just now:
This first is a Peru BCC redtail that will be 2 years old this September... a big guy, well over 3' long..
.

.
Next is a Crawl Cay, Belize Boa c. ssp. at age 21 months (1 year 9 months), still fits in my hand.
.

.
And last is a Tarahumara, Mexico Boa that will be 3 years old this July. Nearly sexually mature and not quite 30" in length.
.

.
... all three of these animals are obviously healthy and properly proportioned. How big your boa may be at a certain age or how large a meal it can eat are not the best way to judge the overall condition or health of your Boas. Each boa is an individual and should be treated that way!
.
Gus/Rio Bravo Reptiles.