It is highly recommended to use a ring sizer or multisizer to find your sizes, I do keep them available in the shop at the underlined link, just give it a click.
For most of my splints, you will be finding the size above and below the affected joint. Place the multisizer on the finger, just like you are measuring for a ring, don't angle it. If you suffer from Heberden's or Bouchard's nodes, be sure to account for that when taking your measurements.
For my large thumb splints that require bracelets, you will be finding the ring size of the thumb. Yes, the splint is worn around the mid/MCP joint; however, it is the thumb ring size that is needed. There are rare instances where I will need additional measurements, and this is only if you are ordering with the assistance of your Doctor, Occupational or Hand Therapist.
As always, if you have any questions, don't hesitate to drop me a line!