How to choose the right mask size
You need to position your mask on your face without the strap, and press with your hands to make sure it’s firmly in place, blocking your breath.
If the mask "suctions” properly, and stays stuck to your face, it’s perfect and means that it is your size and will be watertight.
If it unsticks and falls off, that means that air is getting through, and so water will get in, so you’ll need to try another size or another mask.