|Measure in a straight line, not along the curve|
The standard size belly ring is 14 gauge. This is the thickness of the bar. A large selection of jewelry is available in this standard gauge. If you are pierced with a thinner or thicker needle, you will only have basic steel barbells to select from. Any professional piercer should pierce for a 14 gauge barbell.
The length is more tricky for the Piercer. The end result can be based on the distance of the piercing and the depth of the piercing. Ideally a 3/8" or 7/16" curved barbell will fit properly in a healed belly piercing.
Now, when I say the proper length is 3/8" or 7/16" (only 1mm difference for these 2 length), I do not mean for the barbell to be completely inside of the piercing. There should be some barbell showing, to allow the piercing to breathe and to provide room for sitting and stretching.
A fresh piercing will either use a circular barbell (best for cleaning the piercing) or a curved barbell that is longer than the standard, allowing room for swelling and cleaning.
Three most common problems: too shallow a piercing, a crooked piercing, or too deep an angle, making the belly ring pull into the belly button.