Seven gallons, probably. Depends though, how much fruit do you want the plant to produce?

You can also prune it and take that "Bush" out of the name >.

I grew four bush early girls in a 25 gallon container a couple years ago. They grew about four feet tall and produced a huge number of baseball-size tomatoes. At their peak, I estimate that each plant had more than 20 ripe tomatoes on it. Since they are determinate tomatoes, the harvest comes in a rush, but the plants continued producing for at least six weeks.

My guess is that you could grow one plant in a 5-gallon pot and get a decent yield if you feed it very well. Seven gallons would probably increase your yield. By the way, I got a Topsy Turvey upside down planter for Mothers Day that year, so I had to use it. I grew one bush early girl in it, and that one only produced about 8 tomatoes, 3 of which were very small. I think it only held about 3 gallons of soil, and it was very difficult to keep watered. Fortunately, the second year I tried to use it, a squirrel attacked it and I was able to dispose of it without breaking any small hearts.