Inner diameter: 2.75"
Outer diameter: 3.325"
If you are using a medium weight fabric, in widths of 26 "-30", you might be more comfortable with the medium size rings. In general, the medium size rings will give you a sling that is secure, but may not be as easy to adjust as the large rings if your fabric is on the thicker side. Many people prefer medium rings when making a single-layer dupioni or shantung silk sling, especially if it will be machine-washed.
For more information: www.slingrings.com
This is for the pair of white (top right in photo).