Your container size really depends on what you’re growing and how big the plant is. (Typically, growers allow one gallon per one foot of assumed finished plant height in peat mix or coco coir pith.) Keep in mind that as long as there’s support to keep the plant upright and space for the roots and stems to grow, pots can be as small as three inches (7.5cm). For larger, more water hungry plants, like tomatoes or peppers, it’s best to use at least a three-gallon container. The smaller container, the more frequent the watering.