In my opinion MOST adult Boas will do fine in a 4x18 or 4x2. (Especially males). Since Boas are semi-arboreal you would get pleasure watching it climb given the opportunity with a 4x2.
However, one of my Boas is thick bodied and 7 feet long. Given that, I felt a 6 foot cage was in order. This one below (the cage on top), is a temporary 6x18 x2 deep. I have a 6x18x30 deep on order. I wish it was 2 ft high, but just could not work it out to do so.
Punch, Wild Type BI in top cage.
My other Boa is just under or at 6 feet long, and this 4X2x2 works quite well for him. The photo is of only half of the cage.
Butch, Wild Type BI