For Mask Size Reference:
Compared to disposable masks, S/M sizes are similar to its unexpanded state, while the L/XL mask have 1.75” inches more wide coverage in comparison, but fit nicely from the nose to just below the chin. If a disposable mask fits somewhat loose, a S/M mask will fit. If the disposable mask is fully expanded or stretched, a L/XL will fit.
S/M fit well on kids, and for adults with smaller/narrow faces or prefer a snug fit. L/XL fit well on round and wide faces, or prefer more coverage.