For an adult male, a 20g long will be fine, but for a very large female (+200g) I would go with a 30g breeder.

Tubs are a great alternative to tank. Tubs hold heat and humidity well and only require the use of UTH's with thermostats. For an adult male, a 32qt tub is fine, and for a big adult female, a 41qt tub is good. Plus, they are so much easier to clean then a giant heavy glass tank.