I need with this it involves aftership returns center?

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I have a question I have aftership returns center I have a USPS account that I got from them in postmen courier section, so that means I don't have a USPS account with the actual company. I did make a FedEx account since I wanted to have my own courier. The question I have is in the "Set up auto label generation" section in the "PACKAGE DIMENSION" There is a length, width and height. I'm dropshiping I'm using eprolo to get products from aliexpress, and they vary from from length, width, height and weight. What should I put under length, width and height is there a universal number I can put there?