Send out sizers are available. To request one, simply email email@example.com - there will be a fee for postage.
If you'd prefer to do it yourself, one of the easiest ways to work out the size you wear is to take a piece of ribbon, string or a strip of paper, wrap it around your finger and mark where it overlaps. Lay the piece flat, and measure to the mark, then consult the chart to find your size.
Alternatively, if you have accurate callipers on hand, you can easily measure the inner diameter of your favourite ring.