The S measures 6.5 inches across, from ear loop to front. The M measures 7 inches. The L measures 7.5 inches.
To get the best fit, I recommend measuring your face from the bridge of your nose to the back of your earlobe. Match that measurement to the mask's, rounding down. I wear a S, and my distance is almost exactly 6.5". Luke wears a M, and his distance is about 7". If you're between sizes, size down: it's better to get a tighter seal than to have it gapping around your face, they do stretch.