I use 3 tub sizes, 6qt, 12qt, and 41qt.

Hatchlings to 300 grams stay in the 6qts, subadults and adult males stay in the 12qts, and breeder females stay in the 41qts. I have a couple breeder males that will need to be moved to a 41qt in the next year or two, but they fit in the 12qt fairly comfortably for now. If they look cramped I move them up a size, if they are poor feeders I keep them a smaller tub. For example, I have a couple 400 gram animals in the 6qts because they don't eat well. I also have a couple 1,000 gram females in 12qts for the same reason.