There are many factors to consider when purchasing a snorkel: shape, mouthpiece, barrel type, valves, and even how well you like the look of it. One factor you may not consider is the length of the snorkel.
Some people might think a longer snorkel is always better. The longer it is, the less chance water will get in, right? While true, there are two other important considerations:
Dead air space is a concept that comes up in many different areas of scuba. Dead air is the gas in airways left from the end of your last exhale. It consists of carbon dioxide, and is blocking the path for fresh air. This gas has to be inhaled first before new air can reach you, yet it is "dead" and thus worthless to your body. The space refers to where this dead air resides.
For snorkels, the longer they are, the more dead air space they have. When you exhale through a snorkel, the carbon dioxide is trapped in the barrel. On your next inhale, you must first inhale all the carbon dioxide from the barrel before fresh air reaches your lungs. The longer the barrel, the more difficult it is to breath.
Clearing gets harder with longer snorkels as well. When water gets trapped in the barrel you must clear it by forcefully blowing out any trapped water. Longer snorkels simply hold more water. Snorkels do have valves on the mouthpiece to force water out, but it will still take a little more effort for longer barrels.
These factors should figure in your purchase of a snorkel. Usually, a snorkel just long enough to clear the top of you head should suffice. These days they even have guards on the end of snorkels to prevent water getting in the top. You'll pay extra for it, so it's something to consider if you think you'll often be snorkeling in choppy waters. Otherwise, you use a snorkel rarely enough to be worth the added expense.