Nipple jewelry has been used to show signs of strength by guards of Roman Caesars. Some historians says that Victorian women would pierce their nipples and hand a chain on them.. Regardless, nipple piercing are not only very elegant piercing
to see, but also rather titillating to the wearer.
The smallest jewelry men should consider is a 14g. Any thinner might grow out or tear and leave a scar if played with too hard.
Women need slightly thicker jewelry because their nipples are not only larger, but also subject to abuse from bras, etc. Generally a 12g is sufficient. Also you might want to consider Barbells instead of Nipple Rings since they put less pressure on your nipples and generally heals quickly.