What primarily influences the size and shape of ULDs?

The size and shape of Unit Load Devices (ULDs) are primarily influenced by the overall dimensions of the aircraft's fuselage. ULDs are designed to fit snugly within the cargo hold of an aircraft, maximizing the use of available space while ensuring that the load can be safely and efficiently loaded and unloaded. The fuselage dimensions dictate the maximum allowable width, height, and length of the ULDs that can be used, ensuring that they fit properly within the aircraft's structure.

While international aviation regulations play a role in determining standards for safety and compatibility, it is ultimately the aircraft's design that shapes the physical constraints of the ULDs. Factors like flight altitude limits and airport runway capacities do not have a direct influence on the dimensions of ULDs, as these relate more to operational considerations rather than the physical design of cargo containers used in air transport.