What is the largest tank size a person should place a Betta to live in?

if you only have a betta then 5 gallons is big enough. well my betta is in my 65 gallon tank and he has never been happier. the filter is strong, but he stays at the other side of the tank away from the current. plus he lives with at least 40 other fish. and he can swim against the current, plus i have plants in the tank that help sort of slow the current down