Asking how big a container must be for a tomato plant to reach fruiting stage is a little like asking how many licks it takes to get to the chewy center of a well known lollipop. The answer seems to be, it depends.
In the case of this Black Vernissage tomato plant, the answer is seven inches across by seven inches deep. That one could be grown in such a small pot is quite a surprise since this variety is considered to be indeterminate in nature. I placed the pot in the bottom half of a plastic gallon jug I had cut in half. This made it possible for a ready supply of water to always be available.
It's always nice to have an unexpected success in the garden.