Well first of all, length isn't really a good measure of how big the snake actually is since getting an accurate length measurement is nearly impossible. Weight, however, is much more informative.

If your snake is full grown, a 32-42qt tub would be perfect. If your snake is younger than a year old, a 15qt tub is probably better.

As long as the tub is short (under 8") it doesn't really matter. They don't really need as much space as you think. Just enough for two hides and a water bowl.