What's the best pot size for a small closet? I am starting a Sea Of Green and wondering what is the optimal pot size? I will be using a 400W HPS to flower approximately 20 plants in a 2 foot x 2.4 foot x 5 foot closet.

You are planning to grow 20 plants in about five square feet, so each plant will have a canopy space of about 6 inches x 6 inches. The best containers for you to use would be 6 inch square containers that are about 8 inches tall. This will provide enough space for each plant to grow a single large bud.