This is my personal preference (and keep in mind that I have never bred, but am starting a nice collection of boas to breed in the future):

I will raise my females very slowly, not forcing any growth on them. And I will NOT breed until the girls are at least three years old. I feel that this gives the girls enough time to get to a good weight, build up some nice muscle mass... and, if I don't feel comfortable at the three year mark, I'll wait another year.

Sure, it's possible the breed them younger and smaller---as is done successfully quite often---but I don't want to risk it. These girls are my pets more than anything. I want them healthy and happy...

Jeez, give them some time to be young and carefree!