What is the largest priority mail box available

According to the United States Postal Service (USPS), the largest Priority Mail box available is the Priority Mail Large Flat Rate Box. Here are the dimensions and specifications:

This large flat rate box is ideal for shipping bulky items, such as:

Keep in mind that the box must be completely filled to take advantage of the flat rate pricing. If the box is not fully filled, you may need to pay additional postage based on the actual weight and dimensions of the package.

It's also worth noting that the USPS offers other large Priority Mail boxes, including the Priority Mail Regional Rate Box B and the Priority Mail Large Box, which have slightly different dimensions and weight limits. However, the Priority Mail Large Flat Rate Box is the largest and most cost-effective option for shipping large, lightweight items.