In Scuba diving or snorkeling, your MASK is the most important piece of gear you'll ever own.  Buy it first.  It's your window to the underwater world.  Scuba is a very visual sensory experience, relaxing, and highly personal.  So, your porthole into the sea must allow you to see without pulling, pinching, or leaking.

Selecting a mask that fits well is key to avoiding anxiety or stress.  The way most shops "fit" you is really the last step in finding your perfect fit.  They usually display a variety of styles and promote special features or package costs or just wear you down "sniffing" style after style.  That's just wrong!

Every skull and face varies minutely in curvature and tissue thickness one from another.  There is no universal fit, but all make an average, as well as oversized, and narrow or kids fit.  The trick is Try Them On.  The curve of the left to right aspect of the mask frame must match the width, flatness, or curve of your forehead.  The up and down curve of your face must match the heighth and fullness of the face.  And it has to accommidate the size of your nose without distorting the facial fit.  Even a wide, soft, pliable, feathered sillicone skirt can't make-up for a too narrow or too short (too Small) mask.  Features like a nose purge can't overcome the water leaks that come with a too Large style.

So, how do I find the right fit?  Step-by-step it's easy:

1.   If in real estate the primary factor is Location, here it's Fit, fit. fit. 
With the strap removed or pulled forward over the lens, touch a mask to your face and look into a mirror (or have a friend look into your eyes) at it's contact with your skin.  Don't push or sniff.  Just look for contact fit and rock the mask left-right and up-down to observe gaps that could be leak points.  If the mask is too narrow, the skirt will rest inside the skull's eye orbits and leak ever time you blink.  If too wide (or a fat face), smile lines can channel air out or water in with every laugh.

2.  After you narrowed the field, go back over your culled choices to compare their beneficial Features. 

Volume refers to how much air (or water) the mask can contain.  A low-volume mask clears easier and faster if flooded.  The closer the lenses come to your eyes, the lower the volume.  Most 2-lens masks are low volume. 
Number of lenses vary from a 1 single pane, 2-lens styles (like eyeglasses), 3-lens (single pane plus side windows), to 4-lens (2-lens plus side windows), plus panoramic styles that fuse or bend the lenses to eliminate the side posts required of 3 or 4 lens to increase width of view.
Lens shapes vary the heighth of view.  Rounds, Teardrops, and others allow a more downward view to help you see where you're walking topside or make gauge reading simpler.
Magnifiers either flat ,stick-on, or angled act as bifocals underwater.  Diopters pop-in for vision correction on some styles or any can have custon ground or bonded lenses.  
Purge valves allow you clear water from your mask by exhaling out the nose.

3.  Colors and light transmission of frames do more that accent your fashion.  Black farmes with opaque black skirts frame the eyes of your closeup underwater photographic model, while translucent colors reduce cloustrophobia in low light conditions and offer contrast or compliment to your BC and wetsuit colors.  Limit fashion colors to three.  Some masks even offer color correcting filters to their lens.

4.  Cost must be weighed against the factors above.  Is a $10 bargain really a deal if it doesnt fit (leaks), shadows anxiety fo what might be just out of view, or distracts from the dive?  When properly cared for masks last years and are well worth the over $100 you'd pay for safety, security, and relaxation of a good fit. 

Finally, keep a comfortably loose mask strap.  Water pressure (or holding a sniff)  will keep it in place.  Pulling tighter can distort the skirt of your mask and cause leaks in an otherwise perfect mask.  Relax and have fun!