What do you need before generating the label?
A UPC-A number registered to your business through GS1, if the label is going on something sold through a retailer or marketplace. See do you need a GS1 number if you’re not sure. For internal stock, any consistent number works and Code 128 is usually the better format.
What size should the label be?
Follow GS1’s magnification guidance where the label is for retail: 80% to 200% of nominal size. Below 80%, our own testing shows the symbol can still be physically readable by a phone, but a retailer’s fixed scanner may not accept it. See the UPC-A generator for the measured detail.
What’s the most common mistake?
Not leaving enough blank margin (the quiet zone) on either side of the bars. A scanner uses that blank space to know where the symbol starts and ends; crop it too close and scans that should work start failing intermittently, which is a harder problem to diagnose than a label that simply doesn’t scan at all.