Ideally, you could get 4' x 2' x 2' enclosures for the corn and the royals, or two 8' x 2' x 2' enclosures to divide up between them. That's the new "recommended" enclosure size for those species in some circles. You could probably get away with using the 40 breeder for the corn or male royal if you wanted to, though. In the PVC case, I wouldn't get anything shorter than 18 inches. It's nice having a taller enclosure come cleaning day, though they're harder to heat with just a UTH.
The boa's a harder call. If she has a good amount of Central American blood in her, she'll probably stay on the smaller side (~ 6 feet) and be happy in a 6' x something. If she has more Columbian blood, she could get larger (~ 8 feet) and need an 8' x something. A 6' x 3' x 2' could be a good compromise between the two. Out of all your snakes, the boa will be the one that needs the room the most, so I'd prioritize investing in her enclosure first.
Sent from my SM-N960U using Tapatalk