Belly button rings, like all piercings, are categorized by gauges. The gauge number stands for the thickness of the ring. When you first get your belly button pierced, you will be pierced with a standard ring, which is usually a 14 gauge. The standard ring is fairly thin, so it goes into your skin as smoothly and painlessly as possible, making only a small hole.
Gauges can range anywhere from zero to 20. The higher the gauge is, the thinner the belly button ring is. The gauge number is equivalent to a specific hundredth of an inch. The standard 14-gauge belly button ring is fairly thin. However, you can ask for a different gauge depending on your desires. It is common for people to change their belly button rings after their piercing has fully healed.
Length is also a factor in belly button ring sizes. Aside from being a 14 gauge size, your initial belly button ring will also be longer in length than other rings. Circular ring lengths are measured by their diameter, and barbells are measured by the distance between the two balls at either end.