Smallest? Are we talking adult length or weight? What are your considerations for getting a boa? I personally am not into the little boas. I would prefer something with some size.
Depends on if you want a red tail boa, or just any kind of boa.
As far as red tails.. a male hog island boa will stay pretty small.. usually comparable to a large ball python, or perhaps a bit bigger. This is my large female hog:
Other dwarf red tails include the central american and nicaraguan boas. I am not a fan of these because they have a rather muddy pattern typically and many are aggressive.
As far as other boas, there's quite a selection. I keep two species of sand boas. These snakes top out at 3' max, but many are quite a bit smaller.
Roughscaled sand boa:
Kenyan sand boas (anery and normal):
Other small boa species that come to mind are members of the candoia genus (viper boa, solomon island tree/ground boa etc) though these are not known for being docile, and can be hard to switch to eating rodents.
Other dwarf red tails include the central american and nicaraguan boas. I am not a fan of these because they have a rather muddy pattern typically and many are aggressive.
Not all of them. Try a Bluefields locality nicaraguan boa.