It is most important that the mask fits properly. Moustaches commonly interfere with the fit. If they are bushy and there is no skin above them under the nose, air will leak through the hair and cause the mask to leak.
Some divers shave the extreme upper part of the moustache.
Some apply silicone grease to the hair. Some shave the entire thing off. Once the mask fits properly using the method described in 1 above, then it is necessary to determine whether the size is correct. It would be hard to get a mask that is too large to fit. If you do use a mask that is too large, the strap would have to be so tight it will cause the mask to leave a red ring around the face.