I have heard many people more qualified than I say what minimum size that snakes are happy in, but I have racks, and I keep snakes as a hobby, not as a business. In other words, I don't need to have a thousand snakes in a certain sized area.
I've seen snakes freak out and they wouldn't stop freaking out until I put them is bigger tubs. After I moved them, they were OK with it again.
I don't know if something was unacceptable with their tub, but I just attributed it to a too small tub. Maybe that's not it, I don't know.
I keep hatchlings in 12qt rubbermaids.
I keep juvies in 28qt rubbermaids.
I keep adults in cb70 tubs, they seem to like the extra space.
I just love the ARS cb70 tubs with the deli cup holder already built-in. Smooth bottoms, no ribs, and rounded corners.