The smallest Oura ring size is US6. If it’s too big for you and you really want to wear the Oura ring, you could try adding some padding (inside and top) or wearing it on your thumb. We generally don’t recommend padding as a long-term solution because it adds a layer of friction to the otherwise smooth inner surface.
The size range for the foreseeable future is US6 to US13. Let us know if there’s a need for smaller sizes and we’ll promise to look into it.