The real answer is a barrel length that allows you to hit the birds. With that said, general consensus is that shooting skeet is a somewhat 'quicker' game than sporting or trap. Shorter barrel lengths are the norm. Anything from 27-30 inches or so is fine, but you want to make sure that the gun is easily swung. Balance in the gun can help as well. I've shot 50 yard crossers with a 34 inch barrel that seemed to almost stand still as I pulled the trigger. It was almost too easy. But that same long, heavy barrel would be tough to shoot a low #6 or #8 skeet station. It's the same principle as golf clubs. The driver is usually the longest club in the bag. Your pitching wedge is the shortest. Nimbleness vs. smoothness over a long wide swing.