A hatchling would be fine in a 6qt tub
A juvinile would be fine in a 15gt tub
An adult male in a 28qt tub
A small adult female in a 32qt
A large adult female in a 41qt

I wouldn't know by length, only by weight.

The maturity is measured by weight, so a scale is a must have.
Also you will need to keep records. How often you snake eats, poops, sheds, its weight every couple months or so.