It is mostly about personal preference. From there it is about what is available. I'd say 90-95% of O/U skeet shooters are using 28 - 30 inch. Some people still shoot 26 and a few are shooting 32 (or longer).
How big of a person are you? Make sure that you will be comfortable swinging which ever size barrel you are getting.
32 isn't too long for skeet. There is a group of pretty solid shooters who have gone with barrels this size. It may cost you a few extra bucks if you tube the gun. Bottom line you can use what ever you are comfortable with on the field.
Brad.