If you want to ensure you get the correct size, you can take measurements, or you can select your size on the left of this chart, according to your height and weight.

This photo shows the two measurements you should take for accurate sizing. The red line depicts half of the circumference starting from the tip of your nose, under your ear and around back to you nose. The black line shows the distance between the top of your nose flat along your nose, down to just under the tip of your chin.