I have owned dozen of trailers over the years. Bumper pull trailers are a lot easier to hook and unhook but that is about the only advantage they have over a gooseneck. I have a 7x14 14k Lamar gooseneck dump trailer and really like it. I put my skid steer in it once (way overloaded) for a five mile trip and it worked fine. If you are going with a 14k trailer I would strongly recommend a gooseneck. Heavy bumper pull loads behind light trucks are scary.
As to length I really like the 14’ but definitely wouldn’t get longer. a few years back I had a 16’ dump and it was far to easy to overload it to the point it wouldn’t lift. The 12’s seem too small to fit various things in and the 14’s seem just about perfect. I haven’t used the dump feature or hauled any dirt/gravel with mine in about six months but I have used it a ton hauling other things. Last week I used it to pick up a grain drill 300 miles away, the week before that I hauled two tons of deer corn to the ranch, I haul my UTV in it a lot, I have even stuffed a army truck in the bed once.