The wearable area is measured from the start of the flare to where an o-ring would set IF you wanted to wear one on the very end. There is no groove on these.
6g through 00g have approx 5/16" wearable
7/16" has a 9mm wearable
1/2" has a 3/8" wearable
Jewelry is measured through a gauge system (see chart below). The higher the gauge number the smaller the wire is. A standard "earring" is usually 20 gauge. Jewelry gauges/sizes can vary between manufacturers. There is no universal regulation on companies to make a standard size in body jewelry. A big part of this is that some jewelry is made overseas and measured in millimeters. And in the USA jewelry is measured in inches/gauges. So many times jewelry can be anywhere from .5mm to 1mm different.