Oura Ring 4 Sizing Kit The Oura Ring 4 Sizing Kit is a custom sizer that allows for you, and those around you, to find the ideal fit before purchasing. Oura Ring 4 sizes are slightly different from standard ring sizes, so it is important to size yourself using our sizing tool and to follow our instructions. A properly fitting Oura Ring 4 will result in the highest possible data accuracy, comfort, and overall experience.