First I wanted a BCI, but it is kind of "big"
Males are usually smaller right? What would be their sizes?

So I looked around more and saw that a "Rainbow boa" is the shortest of the big guys, right?

What would be minimal enclosure?
How is their girth compared to the others?
I can't tell by pics. I just don't want it to be a cornsnake girth type.

Basically I'm looking for 5-6 (taking a few is fine)feet.
So a male of which?