External circlips fit snug around the shaft and press against it instead of outward, unlike the internal circlip.
The best way to find the external circlip size you need, is to measure the outside diameter of the shaft (not the circlip groove). For example, a 30mm shaft will require a 30mm circlip (keeping in mind that the inside diameter of the circlip will measure less than 30mm as it has not yet expanded).
If however, you want to measure the circlip itself to see what shaft it suits, the correct way is by measuring from the outside on one side, then to the inside on the other.