For small family gardens in urban settings, the most popular and practical planter box size balances ample growing space with spatial efficiency. Dimensions around 24 to 36 inches (60-90 cm) in length, 12 to 24 inches (30-60 cm) in width, and a depth of 12 to 18 inches (30-45 cm) are widely favored.
This size range is ideal for several reasons. The depth is crucial, as 12 inches accommodates herbs, lettuces, and most annual flowers, while 18 inches provides necessary root space for vegetables like tomatoes, peppers, and beans. The compact footprint allows for flexible placement on balconies, patios, or small yards, often in a modular arrangement. It's large enough to support a meaningful family harvest yet small enough to manage easily without overwhelming a limited area.
For accessibility, especially for children or those using raised beds, a height of 12-18 inches is also standard. Ultimately, the "best" size adapts to your specific crops, available space, and whether you'll use single containers or group several together for a cohesive garden feel.