One online jewelry site has come to a conclusion that fingers come in three different shapes. Perhaps knowing your finger shape may help you in sizing your rings correctly.
Thin fingers are hardest type of fingers to wear a ring on. The problem with thinner fingers is that often after putting the ring over your knuckle, your finger is much thinner below it and the ring tends to slide and spin around on your finger. Talk to your jeweler about your best option for rings with this type of finger.
Medium sized fingers have knuckles that are just a hair larger than the part below them. Most people with medium sized fingers shouldn't have a problem with ring being loose and spinning around. The ring should fit snug and comfortable next to the skin.
Large fingers also tend to have a small problem when wearing rings. People with large fingers generally have a finger area under the knuckle that is larger than the knuckle. There have been instances when people with larger fingers have had rings slide up over the knuckle and fall off due to this situation. Check with your jeweler on what is best for you if you have larger fingers.