Postal Letter

What are the dimensions for the biggest USPS Flat Rate Box?

What are the dimensions for the biggest USPS Flat Rate Box? I need to ship out a Wii Fit BTW I didnt know which category this belongs in lol

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1. 12” x 12” x 5 1/2” https://shop.usps.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10001&storeId=10052&categoryId=10000036&langId=-1&parent_category_rn=10000002&top=&sort=&viewAll=Y link for the sizes