When creating the barcodes, the Bar Width setting equates to the mil size. Bar Width specifies the width of the narrowest element, whether it's a bar or space, in thousandths of an inch. So a bar width setting of 5 means the narrowest bar or space in the barcode will be 0.005", making it a 5-mil barcode.
If you are creating the barcodes in Excel using the Create Font Barcode, there is no setting for bar width, only font size and density. For those settings, High density = 15, Medium density = 20, and Low density = 36.