For a small city balcony, the best planter box size is usually a long, narrow rectangle rather than a deep, bulky cube. Aim for about 24–30 inches long, 10–12 inches wide, and 10–12 inches deep. This size fits most balconies, leaves walking room, and holds enough soil for herbs, leafy greens, peppers, and compact flowers. If your balcony is very tight, choose 18–24 inches long, 8–10 inches wide, and 8–10 inches deep; just avoid anything under 8 inches deep unless you are only growing shallow-rooted herbs like chives, parsley, or lettuce.
Depth matters more than length for plant health. For herbs, lettuce, and annual flowers, 8–10 inches of soil depth is enough. For tomatoes, peppers, eggplant, or dwarf shrubs, choose at least 12 inches deep—ideally 14–18 inches for one large fruiting plant. A 24-inch-long, 12-inch-deep box can often hold three to four herb plants, two pepper plants, or one compact tomato with a companion herb.
Width should be comfortable to reach across. In a small city balcony, 10–12 inches wide is the sweet spot: wide enough for roots, narrow enough to save floor space. A 30-inch-long box may be better along a wall or railing; a 24-inch box works well in corners. If you use railing planters, look for 24–30 inches long and 8–10 inches wide, with a secure bracket and a depth of 8–10 inches.
Also consider weight. Wet soil is heavy, so a 24 x 12 x 12 inch box can weigh 60–100 pounds or more. Use lightweight potting mix, not garden soil, add drainage holes, and place the heaviest boxes near the wall or over structural supports. Self-watering planters can be helpful, but they add weight and cost.
Final recommendation: for most small city balconies, choose a rectangular planter box around 24–30 inches long, 10–12 inches wide, and 10–12 inches deep. Scale down to 18–24 x 8–10 x 8–10 inches for tight spaces and shallow herbs, or up to 12–18 inches deep for fruiting vegetables. Measure your balcony, check the weight limit, and pick the largest box that still lets you move, water, and enjoy the space.