We go by a length to floor space ratio... 1ft of snake = 1sqft of floor space. The numbers below are the approx floor space of Sterilite tubs (we go directly from 6 to 27, since they usually grow too fast to necessitate the 16qt step), so you can get an idea of when we trade up:

6qt = .75 sqft
16qt = 1.4 sqft
27qt = 2.7 sqft
41qt = 4 sqft