On a large rectangular planter the length is what you are actually buying. Depth and height are usually constant across a family, so the decision is how much continuous run you need and how few joints you can accept. A single longer planter almost always reads better than two shorter ones butted together, because the joint is the first thing the eye finds along a straight line.
Size brings the material question with it. At this scale a cast concrete trough needs equipment to place and cannot be moved once filled, which is why fiberglass dominates roof terraces, podium decks and frontages where access is limited or the structure has a load allowance. The planter's own weight is the smaller half of the total in any case: soil is the heavy part, and a longer planter holds proportionally more of it.
Read the interior length rather than the exterior when planning planting, because the wall thickness is included in the outside figure. The largest sizes ship by freight truck and are delivered curbside; smaller sizes in the same family often ship free by standard ground carrier.
