Under Woocommerce > Settings > Shipping > USPS you see there are default fields for: Custom Box Height, Custom Box Length, Custom Box Width, Box Girth, and Box Weight.
In my shop I set those to the average settings for my products and my USPS works fine. I haven’t tested it, but I am thinking that if there’s a product that’s much larger or much smaller and you set the dimensions per product, it would override those default settings.
