Bentwood rings are made to a specific size during construction, and unlike metal rings, they cannot be stretched significantly. The laminated wood structure is formed around a core in the exact size you order, so getting the fit right from the start is important.
The most reliable way to determine your ring size is to visit a local jeweller and have your finger measured with a proper sizing mandrel. Ring sizing kits are also available online, though they are less accurate than a professional measurement. Avoid sizing by measuring an existing ring, because rings can be stretched or resized after purchase, making their internal diameter unreliable.
If you need to resize a bentwood ring after purchase, the workshop can make adjustments within a range of about half a size up or down. Going larger requires adding a veneer layer during reconstruction. Going smaller requires trimming a layer. These adjustments are straightforward but require sending the ring back to the workshop.
A well-fitted wood ring should slide over your knuckle with slight resistance and sit snugly at the base of your finger. It should not spin freely, but it should not feel tight enough to leave an indentation after removal. If your finger size fluctuates with temperature or activity, order a size that fits comfortably during your warmest, most active conditions.