It seems that a lot of people use the following guideline:
6qt for hatchlings
12qt for juveniles
30ish (depends on the tub) qt for adult males, mid sized females
41qt for large females
Or so I've seen.
Usually thats something allong the lines of up to 200g, 2-500g, 500-2000g, 2000+g
As for me: I use a 12qt for new snakes, then throw them into a 32qt, since I dont have any snakes large enough to warrant a CB70 I havent moved up there yet, but pretty soon I'm going to build a hatchling/juv rack and then a CB70 rack.