Internal circlips are retaining rings that fit inside a cylindrical bore or housing and push in an outward motion.
The best way to find the internal circlip size you need, is to measure the inside diameter of the bore (not the circlip groove). For example, a 30mm bore will require a 30mm circlip (keeping in mind that the 30mm circlip will not measure 30mm on the outside diameter as it is not yet compressed).
If however, you want to measure the circlip itself to see what bore it suits, the correct way is by measuring from the outside on one side, then to the inside on the other.