For a small apartment, the good news is that urban planter boxes are usually more compact than people expect. Most standard urban planter boxes range from about 12 to 36 inches long, 8 to 12 inches wide, and 8 to 12 inches deep. The smallest balcony-friendly models are often 18 to 24 inches long, which is a practical size for a compact balcony, window ledge, or corner.
If you are growing herbs or salad greens, a 10 to 12 inch deep box is usually enough. For tomatoes or peppers, choose closer to 12 inches deep and at least 18 inches long. Window boxes tend to be 24 to 30 inches long but only 6 to 8 inches deep, so they suit small herbs and flowers rather than deep-rooted vegetables.
Larger urban planter boxes can reach 36 to 48 inches long, but they become heavy once filled with soil. In a small apartment, measure your space first, leave room to move around, and check the weight limit of your balcony. A compact 18 to 24 inch planter box with drainage and a saucer is often the sweet spot: big enough for a useful mini garden, small enough to fit apartment life.