I followed the sizing guide and was confident I needed a large, since the distance I measured following the instructions was well over 4.25". Sadly this didn't work out for me, the mask is way too large and most of the airflow went right past it. Maybe a better way would be to state the size of the mask and include instructions on how to measure.
The material feels very comfortable and the overall look is great though, so I resized the mask myself. Now the fit doesn't look as elegant anymore, but it definitely works (I can see the mask go up and down with my breath). The material is very breathable and it's overall very comfortable to wear.