Our standard UPC code size is 1.25”x.5”- if you are having someone else do the design, it’s better to simply leave a space. The UPC code will include it’s own white box, so there’s no need to include one in the art (and having a white box in your art will actually make it harder for you to line up the code correctly, as you can’t zoom in during the placement stage, so there’s a good chance of ending up with offset corners). If you simply leave an obvious space for the code, that will make things far easier for you.