This little fellow can cause an oil drip which tracks back down the engine as you ride;
You can clearly see the size marked on it;
12 x 18 x 3, and there’s no spring on the inside unlike some other shaft seals:
The Honda part number is 91201-951-003.
The depth of the seal isn’t critical, so you can use a 12 x 18 x 4 or 12 x 18 x 5 without issue.