The starter motor reduction gear has two washers, one on either side. They are part number 90451-708-000 for all models – CX500, GL500, CX650, GL650, CX500 Turbo and CX650 Turbo – and you can still find them on eBay from time to time if you have lost yours.
Now, we aren’t expert metrologists – this is done using a consumer-level digital caliper set so take the measurements with as many grains of salt as you feel are appropriate. That being said, here’s what we measured the dimensions of the starter reduction gear washers to be: ~1mm thick:
~17.41mm outside diameter:
~12.35mm inside diameter:
To prove that this isn’t perfectly accurate, the best we could measure this aspect of it resulted in a 2.67mm reading:
12.35mm + 2.67mm + 2.67mm = 17.69mm outside diameter, where we measured 17.41mm outside diameter. Given that the calipers are rated to 0.01mm accuracy this is probably down to technique (or perhaps not using vernier calipers to measure rounded objects…). This should be enough of an accuracy level to confirm whether that random washer you found underneath the motor is, in fact, the right one for your starter reduction gear, however!