What is the largest size box USPS will take?

Related questions

1. Where can I drop off USPS packages?
2. How do USPS flat rate boxes work?
3. How much does it cost to mail a 6x9 padded envelope?
4. Is the price of stamps going up in 2019?
5. What is considered a large package for USPS?
6. Is there a weight limit on USPS flat rate boxes?
7. Is it better to ship a large box or two smaller boxes?
8. Is it cheaper to ship UPS or USPS?
9. What is considered an oversized package by USPS?
10. Can I use any box for USPS Priority mail?

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15. #combined
16. #mailpiece
17. #length
18. #retail