I'd recommend spending a little extra and actually going to one of the dive stores and try out a number of masks. Mask fit is like skates or ski boots, one size does not fit all and it's fairly custom as people have different face shapes.
You'll hate snorkeling if you constantly have a bit of a leak because there is a poor seal or because the snorkel rubs your gums. Another consideration is the type of snorkel. Some of the cheaper ones are just basically a tube and there are no features. Things like a purge value at the mouthpiece makes for easy clearing when you get water in the snorkel and some of them have a bit of a "cover" over the top so if there are waves you're less likely to get a mouthful of water.
Once you get a kit which fits well then you'll be more comfortable and can snorkel for hours.
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