Is there a standard barcode label size?
No single standard. GS1 specifies a magnification range, not a fixed size. For UPC-A and EAN-13, that range runs from 80% to 200% of nominal, which works out to roughly 30 mm to 76 mm wide including quiet zones (a nominal 100% UPC-A is 38.0 mm wide). The exact choice depends on how much room the packaging gives you.
What happens if you go smaller than the standard range?
It may still scan on a phone camera, but a retailer’s fixed scanner is more likely to reject it. Our own testing found UPC-A and EAN-13 physically readable by a phone down to 50% magnification, well below GS1’s 80% floor; see the UPC-A generator for the full data. That’s useful for internal labels; it’s not a substitute for staying in spec on anything going to retail.
What size should an internal or warehouse label be?
Whatever fits reliably on your equipment. There’s no registration or spec to follow. Code 128 tolerates more variation than the retail formats; the main constraint is your own scanner’s working distance and the label stock your printer accepts.