Summarize this article:

Opt for a longer glove. Avoid the extra steps and costly seconds that it might take to catch fly balls or make diving catches with a short glove. Adult sizes for outfield positions range from 12-15". Pick a glove whose design covers all of your hand for greater support. Sample a closed-back glove that offers a hole for your index finger to protrude from the glove for additional support and judge whether your prefer this option or not. Opt for the additional support that a closed web offers when catching fly balls and line drives.

Summary:
Choose a size. Choose a closed back. Choose a closed web.