Size of large priority mail box

The size of a Large Priority Mail box, also known as a "Flat Rate Box", is:

This box has a flat rate shipping cost of $14.35 (as of 2023) to most US addresses, regardless of the weight of the contents. It's a great option for shipping items that fit within these dimensions and weigh up to 70 pounds (31.8 kg).

Keep in mind that the box is designed to be sturdy and can hold a significant amount of weight, but it's still important to ensure that the contents are properly packaged and secured to prevent damage during shipping.