Summarize the following:
If you're going to be switching between infield and outfield positions, pick a size as Goldilocks would: not so long that it would impede quick ball-to-hand transfers, but not so short that it would make catches in the outfield too difficult. Keep your wrist uncovered. Allow yourself greater mobility while playing infield positions. Since the open back design sacrifices support for more mobility, avoid gloves with an open web design. Make up the difference with a closed web, which offers more support.
Choose a size. Go for an open back. Choose a closed web.