The general consensus is length+depth=the snakes total length. And as for how long a 36x18 will last a boa, it really depends how much you feed it and also on its genetics but a rough guess from me would be about 2-2.5 years maybe 3 years. I wouldn't bother with a 20 gallon. That is a waste and it will outgrow that in a year. Might as well just start off with a 40 gallon or better yet, start off with a 48x24x14 minimum. I got my 6' BCI girl in one at and she is happy enough although I will be upgrading later to something like a AP T-13(48x30x18) or going for something in the 5-6' range like an AP T-25. And by baby boa, you mean something around 6-8 month old? That is usually about the age I see them for sale or at least that is the age you want to buy them. Give them plenty of time to at least eat 4-6 times on their own and have a couple sheds. You really don't want to chance it with a really young boa especially if you are dropping $400-$800 on one.