The short answer: if you mean a planter that must slide straight through without tilting, the widest urban planter box is usually about 30 inches (76 cm) for a 32-inch doorway and about 34 inches (86 cm) for a 36-inch doorway. That leaves 1–2 inches of clearance. If the planter is rectangular and you can turn it on its side or angle it, the long side can be much wider—often 48–60 inches (122–152 cm)—as long as its depth is no more than about 28–30 inches (71–76 cm) and its height is under about 78 inches (198 cm).
Standard doorways vary. In North America, interior doors are commonly 32 or 36 inches wide and 80 inches high. The clear opening is smaller because of the door frame, stop, hinges, and hardware—usually 30 to 34 inches. In the UK and Europe, common internal doors are 762 mm or 838 mm wide. Measure the actual clear opening before buying.
For an urban planter box, focus on the cross-section that has to pass through the door: depth and height. A good practical size is 48–60 inches long x 18–24 inches deep x 18–24 inches high. This fits through most standard doorways when rotated, and it still gives you a generous planting area for a balcony, patio, or entryway. If you want a square planter or one that rolls straight through, keep the width at 28–30 inches for a 32-inch door, or 32–34 inches for a 36-inch door. Remember that casters, handles, and a pre-planted soil load can reduce your clearance and make maneuvering harder. When in doubt, choose a planter that is 2 inches narrower than the clear opening—or plan to assemble it on site.