General rule of thumb is 3/4 as long as the snake.... But to make it more simple, most male BP's can be kept in either a 20 gallon long, or 30 gallon (same floor space) their whole life.

If you're going to use plastic instead, then a 15qt tub can be used for the first 9-12 months, then a 32qt tub is perfect for a male for the rest of its life.