How do I measure a part to find the right seals?

I have a part for an RC car that I want to get seals for. Specifically, I'm trying to seal the area outside of the ballbearings. Note the grove in the photos.

For example, one of the bearings has an outside diameter of 21mm. The groves all have a width of 2mm and a depth of approximately 0.75mm deep. Giving these measurements, what should the seal size be?

 

I'd pack it with green slime or marine grease

 

Orings?

 

You'll need to know ID and OD. Once you have that, you can find an o-ring that size.

 

Should the oring size match exactly or should it be oversized or something like that?

 

Just a little snug would be my guess.

 

They run slightly oversized already, so they fit snug in the groove.