Find a Large Pot. Even small tomato varieties have substantial root mass. The minimum pot size for a patio tomato should be 14 inches in diameter, with a capacity of at least 5 gallons. For best results go with a 17- to 20-inch pot that holds 15 to 20 gallons.And try to avoid black; it may cause the soil to overheat.ind a Large Pot. Even small tomato varieties have substantial root mass. The minimum pot size for a patio tomato should be 14 inches in diameter, with a capacity of at least 5 gallons. For best results go with a 17- to 20-inch pot that holds 15 to 20 gallons.