The size of the metal post required for a 20x24 carport depends on several factors, including the design of the carport, the local building codes, and the specific load requirements. It's always best to consult with a structural engineer or a professional builder to ensure that you choose the appropriate size for your specific circumstances.
However, as a general guideline, you can consider using 4x4 inch or 6x6 inch metal posts for a typical carport structure. These sizes are commonly used for supporting carports and provide adequate strength and stability. The exact size will depend on factors such as the height of the carport, the weight of the roofing material, the local wind and snow loads, and other design considerations.
Again, it's essential to consult with a professional who can assess the specific requirements of your carport and provide accurate recommendations for the size of the metal posts. They will consider the local building codes and ensure that the structure meets safety standards.