tub sizes are measured in quarts. The commonly sized tubs for different sized ball pythons are as follows:

6qt: under 300g
12-15qt: 300-600
28-32qt: 600-1500
42qt: 1500+

Most people who use tubs use them in a rack setup with flexwatt heat tape. This can be ordered online but you need to wire it yourself (unless you pay someone to wire it). You can also use a regular UTH that you would use for a tank.

With either method you MUST use a reptile thermostat to control the temperature of the heating element.