They do get large when talking about toads. They would be good in a 20g i would think. But beware, they can get to 6 pounds. And like i said, they do not need much water. You could just do a large Tupperware bowl with water. I would do substrate. Gravel works, so do wood chips. DO NOT DO SAND, it sticks all over them. Remember that these toads do have health risks. They do secrete poisons through there skin and you need to make sure to wash your hands after handling. Also remember that toads/frogs have a defence that when they are picked up they spray pee. It is not dangerous but be prepared for it. If your a true toad lover you will not mind.