The gauge size for a nipple piercing can vary depending on the size of the nipple and the preference of the piercer. However, the most common gauge size for a nipple piercing is 14 gauge (14G) . The gauge size refers to the thickness of the jewelry, and it is important to select the correct gauge size to ensure a comfortable fit. The length of the barbell used will depend on the size of the nipple and the piercers preference, but it can be anywhere from 3/8-inch to 1/2-inch long. It is always best to ask your piercer to be sure about the size and length you need.