Gauge, stem thickness, whatever you'd like to call it, it's the trickiest part to determine. The only sure way is to use a digital caliper, a worth while investment if you've multiple piercings.

If not, grab yourself a ruler with millimetres on. Place the stem of the jewellery over the ruler and it will give you an indication of thickness. Standard sizing of body jewellery 1mm, 1.2mm or 1.6mm.

The same as straight bars but make sure you measure on the side of the curve.