Short answer: there is no single largest size, because the limit is set by your building’s freight elevator and door. For a typical freight elevator with an 8 ft wide x 10 ft deep cab, a 6–7 ft high door, and a 6 ft clear door opening, the largest single-piece urban planter box that usually fits is about 6 ft long x 4 ft wide x 3 ft high. If the elevator is smaller, plan on something closer to 4 ft x 3 ft x 2 ft. If the planter can be assembled on site, you can go much larger: move panels, liners, and soil separately.
Measure these before buying: clear door width and height; cab width, depth, and height; diagonal clearance; turning space in the lobby and hallway; elevator weight capacity; and threshold or pallet-jack clearance. Leave 4–6 inches of clearance on every side. A planter that fits the elevator floor may still fail at the door, and a long box may need to go in diagonally.
Weight is often the real limit. Wet soil weighs roughly 100–120 lb per cubic foot. A 6 x 4 x 3 ft planter holds 72 cubic feet, so soil alone can be 7,200–8,600 lb before you add the container. That can exceed many freight elevators once you include the planter, pallet, and people. Use a lightweight soil mix, a modular planter, or a roof-assembled planter. For most urban projects, the practical choice is a modular planter or a single-piece box no larger than about 6 x 4 x 3 ft, and only after confirming elevator capacity and door clearance with the building.