The most reliable way of finding your ring size for a particular finger is to have a qualified person at a jewellers and ask if it is possible size the finger for you. Alternatively you can purchase a ring sizer from a supplier such as Amazon for a very cost effective price. They are cheap and accurate and quite easy to use and available to you for future purchases.
Measure the ring size when your body is at room temperature ie you are not feeling either hot or cold as your hands actually expand and contract in hot and cold situations.