The best thing you can do is shoot a variety of different guns to see what works best for you. If you go to a large shoot, there are often vendors selling guns and you can test drive several.
The most popular combination is a 34" single barrel and 32" OU. But there is more to it than just barrel length. Do you like light barrels or heavy barrels? Do you need a high shooting gun or a flat shooter? These are all questions that can be answered after you shoot different guns. What works for me doesn't necessarily work for you.
i tend to stop the swing short on shorter-barreled guns. so now i shoot a 34 in bt-99. but i only learned that by shooting a lot and paying attention over time. And having to sort out trends from ups and downs -- hence i second the advice in previous posts above -- shoot a lot with what you choose and see how it comes out and if you have to change later the so be it. If you can try out examples of the candidates great.
As Albert Einstein would say "it's all relative! I tend to like 30 in. barrels on my Model 12s but 28 In. barrels on my 1100 and 870s. On Over/Unders I prefer the 30 in. tubes. You should visit some shops and try the feel of a few guns before you decide. I knew one guy who was doing really WELL with his Browning O/U with 28 in. barrels. He just couldn't wait to get a 32 in model. He tried one and he couldn't hit squat with it! It has all to do with balance, feel, and what you feel comfortable with!
I am 5'8" and shrinking.....I shoot a CG 34" unsingle Trap and love it. I like the inertia and sighting plane of the long barrel. Put 150 rounds through it yesterday.
My skeet gun is a 32" 20g CG Impact.