I went to our warehouse to get the Hopkins trailer wire connector # H20046 to provide you the measurement you requested.
The outside diameter is 0.61 inches using a digital caliper.
A rubber grommet you should take a look at is the Optronics flush mount rubber grommet # A11GB. I went to our warehouse again to measure the inner and outer diameter. The inside diameter is 0.61 inches but can stretch to 0.74 inches. I think this would be perfect for what you are needing. The outside diameter... view full answer...