S & M Retaining Rings offers a useful guide on snap ring sizes, available on its website at SMRetainingRings.com. There are many different styles of snap rings, so it's imperative to understand how they all work, to ensure the proper fitment for the application.
There are two primary styles of snap rings: internal and external. Internal snap rings are designed to go along a groove inside a shaft to hold a bearing or similar object inside. These snap rings use outward pressure to stay inside the shaft. External snap rings are designed to fit on the outside of a shaft and prevent a wheel or other similar object from sliding off the shaft. These snap rings press inward and squeeze against the shaft to stay on. There are also many other variations of these two basic designs, depending on the application.