These are the little 13.8 mm tiny ones (most commonly used on the power switch knobs), not the larger 20 mm C_53 type used on most steam and water valve knobs.
These are also exceedingly difficult to install. We recommend spreading the ends out and using a rubber mallet to pound them into place. Not fun, 9/10 would not do again.
13.8 mm long, 5 mm wide, 7.4 mm tall.
WARNING: This product can expose you to chemicals including lead, which are known to the State of California to cause cancer, birth defects, or other reproductive harm. For more information, go to www.p65warnings.ca.gov.