I think you might be overstating how big a regular sized BCI gets. Even a full grown 6 foot female BCI only needs a 4x2x1 enclosure. If you got a male they would max out in the 5.5 foot range, and this would take 4 years to achieve. A dwarf boa will be smaller but not by much. Maybe 20% smaller. People overstate how big boas are routinely. The internet tends to consolidate pictures of giant 9-10 foot boas, but these are unique rarities.