We have a 16' dump trailer, and the length is nice. We use lots of 16' material and it is great to be able to just throw it in without cutting or breaking it.
Yep, even my 12 ft is easy to overload. I hauled a load of shredded topsoil home last week for my garden and even that was overweight, and there was room for more. A legal load of pea gravel is not much gravel at all.Only problem with longer dumps is loading stone and fill. Easy to overload.