It’s beyond important that your mask fits just right because a correct fit can help avoid leakage and CO2 inhalation (more on that later). Needless to say, you should be able to adjust it using a wide head strap with touch adjustment buckles.
To test its fit, wear the mask with the strap around your head, and exhale through your nose. If you managed to expel the exhaled air quickly outside of the mask, that’s an indication that you’ll manage to expel the water outside too. If not, clearing out the mask will be a challenge. Finally, put your mask on for several minutes to notice if it applies too much pressure on your face, causing any discomfort.
It’s of great importance that you don’t buy any off-brand knock-offs. They can allow water leakage, obstruct your breathing, and cause CO2 buildup. The latter can be fatal and is rumored to have caused some snorkeling deaths in Hawaii by ROPE, which is a condition that arises from breathing resistance.
To avoid this dilemma, find a full-face mask without much air resistance. Some known manufacturers test their masks for CO2 levels, though not many, such as SEAC and Head/Ocean Reef, so get your hands on one of those if you can.
Related to our previous point, find a “real” snorkeling mask, rather than a cheap supermarket bought-on-a-whim one. Plastic glass, used in masks, is not advisable because it fogs up quite frequently, thus undermining your ability to view your surroundings.
As for regular glass, it may break due to an accident, but if not, it’ll surely break due to the water pressure when scuba diving, which can cause severe injuries. This leaves us with tempered glass, which is indeed your safest bet, as it doesn’t fog up much, and it’s shatterproof.
As a rule, the smaller the volume, the better, because there won’t be more trapped air inside your mask than needed. It also makes it that much easier to clear out water if it penetrates the mask and to equalize if you dive below the surface.
Furthermore, it sits closer to your face, decreasing the amount of drag in the water. Still, this is a matter of preference, with some snorkelers preferring bigger volume masks.
Generally we say go for quality over a budget option when it comes to full-face snorkel masks. While not imperative that you tick all of these boxes, make sure that you pay attention to the key aspects of fit and safety before you make your decision.