There's numerous factors that aid in boa growth. Ssp. being a biggie. Feeding frequency, sex, cage temps, genetics all play a role as well. If you are "too" concerned on size, then maybe a boa isn't the best choice. But that's just my standpoint. When acquiring a snake, if you are concerned on it getting too big, then it's not the pet for you.
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Some of the Island ssp. of boa stay very small. A Cayos de los chochinos (Hog Island) might be a good choice for you, male or female.
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Best of luck.