I use sterilite 27Qt. tubs for my adults, 12Qts. for my sub-adults, and 6Qt. for my neonate and yearlings. I have Rosy Boas though. shouldn't be much different that your sand boas.

At some point I may go up a size for the adults (cannot remember the next size up right now) that are about as long and wide but are deeper. Once I have more room for my collection.