I'm not sure if they're the smallest boa but male kenyan sand boas(EASB)tend to be around 15-20 inches. There are several other types of sand boas and I don't know what the sizes on those are. I'm sure someone else can let you know.

I don't know what size the enclosure you are talking about is but adult males can be kept in 12-16q sterilite/sweater boxes relatively easily.

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1.1 Anery KSB
1.2 Snow KSB
1.2 Normal KSB
0.1 P. Albino KSB
1.0 Albino KSB
1.0 Rufescen KSB
1.1 Snow Albino King
1.0 Cali Banded King
0.1 Jungle Hypo Boa
0.1 Golden Retriever