Cool cotton - strap under chin with velcro closure.  As a guideline only:

XS - 15cm width suits Chihuahua

S - 16cm width suits small maltese, poms, cavs,

S/M - 17cm width suits malt/shitzui cross, snoodles, cavoodles,

M - 18cm width - cocker spaniel,

L - 20cm width suits staffies,

XL - 22cm width suits labs, boxers,