If you are on the cusp of one size or another, I suggest you go larger. The mask effectively cups around the chin and the ear straps are adjustable. Due to the special fabric treatment (I suppose), the breathability is a little less than my paper masks, so having a little more room would feel a little better to me. But I still wear these whenever I go out. I have one in my car, and one at my door. They wash easily (by hand only) and air dry pretty quickly.
Oh... and if you enter BUBBLE10 you get 10% off. :) That’s because Andy Slavitt (Obama administration head of Medicaid/Medicare) has ads for these on his podcast “In the Bubble” and worked out a sponsorship deal with Livinguard I think.