I have a 20 foot Wilson. I pull it with a Ford super duty with a 7.3 power stroke. I have hauled 12 800 pound heifers (9,600 pounds) up over a pretty good mountain pass. That was all I wanted coming down the other side. A month ago I hauled 10 cows that averaged 1,250. Coming down off Ladd Pass they all decided to dance. It wasn't a lot of fun for a couple of minutes.
My neighbor who is calving out about 1,100 cows right now has several trailers. They are all 20 foot. There are very few times I would want more length. There are other times if I could fit more cows in I would and I would regret having that much weight pushing me.