How deep (or tall) a raised bed should be depends on the rooting depth requirements of the plants you wish to grow, and whether the ground below the planter is made of healthy soil. Deep rooted plants such as tomatoes, zucchini, corn, and perennial fruits and vegetables, do best in planters at least 22 inches tall, or shorter if the ground below is usable. Shallow rooted plants such as lettuces, greens, annual herbs and most flowers can be grown in planters 11 to 16.5 inches tall, though even these plants may grow larger if given more soil.
Taller planters also provide easier access and reduce the need for bending over. If accessibiliy, rather than root space, is your main concern, you may also want to consider our standing planters. The planters with legs raise the garden to a comfortable working height, without the costs and soil requirements of similar height raised beds.