What size fish tank would a Cory catfish need?

Technically, a 2 gallon tank for one Cory cat, 4 gallon tank for two Cory cats, and so on. The rule is one gallon per one inch of fish and Cory cats can grow to two inches, although the bigger tank the better. I would DEFINITELY not recommend a 2 gallon tank with one Cory cat. They like to be in groups and will be happier and healthier.