The size question is a vexing one. There are no metrics to go by. That is, no measurements or size charts are reliable in making the choice, so none are published. In our decades of experience with customers, we've found roughly 70-75% of the population will fit a Medium size respirator. If you have what you feel is a notably small face (think petite woman) then Small may be the right choice. On the other hand, if your face is quite large (notably so), then a Large may be best.
The best way to actually determine fit is with all 3 sizes in front of you, with preferably more than just one brand choice, and try them on. Then a fit test by one of the common means; Smoke, Saccharin/Bitrex, or banana oil (Isoamyl Acetate). It helps to have a person experienced in this to assist you, but it is not absolutely necessary. Again, for most people a Medium mask will work. Check out this article for more information on how to choose the right size respirator for you or give us a call at 800.829.9580.