5 gallons is about the practical minimum for a tomato in a container determinate or not.

There are very small determinates (patio type tomatos) that can be comfortably grown in less than 5 gallons, but most determinates get to be fairly large plants (3-4') and bushy and use a pretty fair amount of water.

In a sense you can fit more indeterminates into a space because you can prune them to control their width/bushiness, but you can't with determinates without sacrificing a lot of yield.