Sleeve sizes are designed to match the Mask size. However it is possible to use a Sleeve that is one size larger than the Mask size, e.g. a MEDIUM Sleeve on a SMALL Mask. Some users find this to be more comfortable if the fit seems too tight (especially across the bridge of the nose).
We do not recommend using a smaller Sleeve than the size of your Mask. The Sleeve is made from a super-soft neoprene core fabric which should not be over-stretched.