It is best to find your size by getting sized at your local jewellery shop.
It is possible to get an idea at home, but at your own risk as it may not be accurate. One way to do this is to take a piece of wire and twist it around your finger at the size you would like the ring; make sure it will slip on and off your finger. Cut wire open and measure. Match circumference measurement to the coinciding letter on the above chart.
Size Small- K
Size Medium- N
Size Large- Q