This butterfly is 18 x 15mm not including bail. I've added a photo of one I made up myself so you can see what it looks like. To inform you this is quite a shallow setting. Add a stainless steel 1.5 mm diamond cut chain as an extra.

Please note these are not my own design and once stock has gone I will no longer be stocking. If you need to practice with a very shallow bezel area these would be idea for you.