Correct Ring Size
Jewellers use a set of metal finger size rings. This is the best way to measure your finger size. The best way to find your correct size is to start with a size that slides easily onto the finger. Then progressively try smaller sizes until you find one which will not go on. Your correct size will be about half to one full size larger.  In general a wider ring may need to be slightly larger, and a narrow ring may need to be slightly smaller. Fingers are a bit larger in warm weather and smaller in cold weather, so take that into account. 

Finger Shapes
A typical or medium finger shape is one where the knuckle joint is slightly larger than the fleshy part above it where the ring is worn. In this case, the ring should be a close fit over the knuckle, but feel comfortable to wear once it is in place.                                                  Well-built fingers owners will find that the fleshy part of their finger is larger than the knuckle joint.  In this case, obviously the ring will slide over the knuckle easily, but there is a slight danger that the ring could slip off. To avoid this, it is better if the ring fits slightly more tightly on the flesh than for people with "medium" fingers. It may be worth a short period of slight discomfort with a new ring to ensure a snug fit ever after, as the flesh under the ring will usually adjust slightly to the ring.
The people who often have the greatest problem with getting a perfect ring fit, are those with slim fingers, where once the ring is squeezed over the knuckle, it is too slack on the finger, and spins round.