What is the largest sized box USPS will take?

I have a 20x20x20 40lb. box to send. Can I ship it USPS or do I have to go to UPS?
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Yes, the US Postal Service will take that. They take boxes up to 70 lbs and the dimensions are well within their limits.
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They will take that for sure, well UPS will. Isn;t usps in the states and ups in canada?
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go to the post office and ask