Your aim is to achieve a neat and snug mask fit, not too loose and not too tight. Many people fit more than one size. So if the mask is still leaking air (especially around the bridge of the nose for a nasal or full face mask) it might be worth making a few adjustments or trying a different mask size.
If you're having issues with your mask, check your user guide to make sure you're fitting it correctly. Also, check for creases. Don't assume if you're male you need a large mask size or if you're female you need a small one. Your mask size depends on the key measurements of your face. Book an appointment to visit a ResMed Store or authorised dealer to make sure you have the right mask size.