Paintball masks vary in sizes, colors, designs, as do faces. Each mask will fit a person different, so choose a mask that you know will fit well with your facial structure. Obviously this is best done if you can try a few on. Normally asking friends to try theirs, at the beginning of the day before they are drenched is sweat, is a good idea.
Comfort usually comes with nicer masks with a soft design opposed to the hard plastic designs of the cheaper masks. The more expensive mask typically use a softer foam, and can even be covered in a micro fleece type of material making them extremely comfortable on and around your face.
The most important aspect of any mask is to make sure you purchase one with a thermal lens/dual pane lens. Doing so helps with the fogging issues that are common in the more basic, single lenses. If you can not see, you can not navigate the field safely nor have any real chance at shooting someone. Second, for beginners, if your mask fogs, and if you have a cheap one it will, your first and natural reaction is to lift the mask up. You will, as many have before you, and many will after you. This is EXTREMELY dangerous, and will result in getting kicked out of most fields. Do yourself a favor... just spend a few extra bucks and get a thermal mask.