NIpple reduction is a common operation and can be done as a solo procedure in the office with just local anesthesia, or can be part of another breast enhancement.
Both the diameter and length (projection) can be reduced, to fit the aesthetics of your breast. The surgery can interfere with breast feeding, so is often delayed until after having children. Because other procedures like, breast augmentation, breast lift or breast reduction, are also more common after having children, the nipple reduction may be performed while under anesthesia for these other procedures.
Below is a link to my blog showing nipple reduction, in conjunction with a breast augmentation revision. Thanks for you question.