Two buckets each. I think I made sure to use 5 gallon buckets for the inner/upper bucket, but the outer/lower could be 6 gallon. Depending on how these plants fare, I want to try cutting the tops of the buckets down to reduce the amount of soil used. As it is, there is a good 12 to 18 inches of soil, and the plants really don't need that much. I think they would wick the water better with less. But that will be an experiment for next year.
http://www.globalbuckets.org/