Without a doubt, fit and comfort are the most important "features" to look for when selecting a mask. While most manufacturers design their masks to fit the widest segment of the population as possible, there's no such thing as a "one size fits all" model. Face shapes are as varied as the people who wear them, so it should come as no surprise that a mask that fits your dive buddy like a glove, when you try it on, might feel like a coffee can strapped to your face.
There are a whole lot of masks out there to choose from, so there's no reason to settle for anything less than the best. But the only way to find that mask with your name on it is to try on lots of different models. So find a dive store with a variety of masks on display and get to work. To check for fit, place the mask on your face without the aid of the strap. The skirt should make contact with your skin evenly without gaps. Inhaling slightly should create enough of a vacuum to suck the mask to your face. It should remain stuck with only a small amount of vacuum pressure without any air leaking in. Then slip the strap over your head to test for comfort. Pay close attention to places where the mask makes contact with your face, like over the eyebrows, on the bridge of the nose, or on the upper lip. These can develop into pressure points that can become aggravating at best and quite painful at worst once you slip below the surface and start experiencing compression.
By following these tips you can be 90 percent sure that the mask you choose is the right mask, but there is no substitute for taking it in the water for a true test. This is where you confirm the mask doesn't leak, and that there are no issues with comfort when you've got a snorkel or reg in your mouth. Check to see if your dive store offers a return policy that will allow you to exchange the mask if it just doesn't work when you hit the water. With you and your dive store working together there's no doubt you'll find a mask that fits.