Most city gardeners gravitate toward a planter box that is about 4 feet long, 2 feet wide, and 2 feet high—often sold as a 4 x 2 x 2 raised planter or raised bed. This size is popular because it holds enough soil for a useful mix of herbs, greens, peppers, and compact tomatoes, yet it still fits on a small patio, rooftop, or community garden plot. It is also deep enough to support healthy roots without requiring an enormous amount of potting mix, and it is easy to reach across from both sides.
For tighter balconies and narrow courtyards, the most common compact alternative is around 36 x 18 x 18 inches, or about 90 x 45 x 45 cm. This smaller footprint gives city gardeners room for herbs, lettuce, and a few flowering plants while keeping weight and soil volume manageable.
In practice, the best size depends on access, sunlight, and weight limits, but if you want the closest thing to a standard urban planter box, choose roughly 4 ft x 2 ft x 2 ft, or scale down to 3 ft x 1.5 ft x 1.5 ft for a balcony-friendly version.