For one tomato plant on a small patio, the best urban planter box is usually about 15–20 gallons, with a footprint around 22–24 inches long, 14–16 inches wide, and 16–18 inches deep. A compact 18 x 18 x 16 inch box also works well and gives you about 20–22 gallons of soil volume. Depth matters more than length: tomatoes need at least 16 inches of soil for healthy roots, and 18 inches is better if your patio can handle it.
If you are growing a determinate or dwarf tomato, 15 gallons is a practical minimum. For a full-size indeterminate tomato, choose 20–25 gallons if possible, because larger plants drink more water and need more root space. Avoid boxes under 10 gallons for tomatoes; they dry out too quickly and stunt growth.
Look for a planter box with good drainage holes, feet to keep airflow underneath, and a food-safe, UV-stable material. A light-colored container helps keep roots cooler on hot patios. Fill it with a rich, well-draining potting mix, leave 1–2 inches of space below the rim for watering, and add mulch to slow evaporation. Place the box where tomatoes get 6–8 hours of direct sun, and add a cage or stake at planting time. For a small patio, one 20-gallon box per tomato plant is the sweet spot: compact enough to fit, but large enough for a healthy, productive plant.