Mil size refers to the width of the narrowest element, bar or space, of your barcode, in thousands of an inch. E.g., a 5-mil barcode is one that has the smallest bar or space set to 5 thousandths (0.0005) of an inch.

To set this size in Labeler, click on the barcode object on your label template to select it, then in the Properties box go to the Barcode tab. The Bar Width setting there is the same as the mil size for the barcode.

Depending on the symbology selected, you may also see Chars per Inch. Some symbologies, like Code 39, have a characters per inch specified in their rules, rather than a minimum bar width. For symbologies that have both settings, you can change either, and the other setting will be adjusted automatically.

Other symbologies, like UPC-A, have very specific rules for the element sizing, and will not present either setting. In those cases, you may see a % of Nominal setting, which will adjust between 80% and 200% of the nominal element size for that symbology.