roughly it's measured in quarts:
6 quart = hatchlings
15 quart = juvenile/small adults (i think that's the size he's using, i have the same tubs with purple locking latches)
32 quarts = adult
40-plus quarts = big adults/breeding
the 7.2 quart tubs you're looking at sounds good for the first few months if u have a hatchling.