Having pulled my share of trailers off road for work, I'll share my .02
It will all depend on where you want to wheel it. If you have trees and rocks on either side, you will hate that extra foot of width.
I wheel a Ramcharger and it's a fat pig. I have to constantly watch everything off road, and it makes the days exhausting on tight trails. It also makes some trails impassable without getting out and filling in washed out areas, trimming trees or moving rocks. A trailer is that much worse, since it doesn't steer without moving the tow rig around.
Everything will be a compromise, but there are some places you absolutely will not be able to fit an 84" wide trailer. The thing that would really concern me, is if you are planning to take it on a lot of roads that were pushed out on the side of mountains for mining or logging and they have a steep drop off on one side, with the other side dug into the hill. It doesn't take much to lose a trailer over the edge of one of the notorious wash outs or narrow spots in those roads. Having it pull your tow rig off the side of the mountain would be a bad ordeal. I think I would rather deal with too much length, causing me to drag the tail over things and maybe having to work it a little more in the tight corners, than having to deal with too much width. The added stability of the extra width would probably only be interesting if you plan on long trips where the wind blows a lot.
You have to realize that cargo trailers are a lot like a giant sail behind your truck. If you lift them enough to get down trails, they tend to be fairly tippy, especially with weight on the top. They also wander about a bit at higher speeds due to the flat floor and the propensity to act more like an airplane wing than a downforce device.
A cargo trailer will be fairly fragile. The better built ones have a pretty good steel tubing frame, but the corners are pretty thin on the front and the sides are very thin. The floors are pretty tough, but they are plywood and dragging them across rocks will destroy them and the repairs would be difficult to make pretty again. The fenders are just sheet aluminum and most trailers have them installed in such a way that they are not easily removable for replacement. Cargo trailers also tend to have barely adequate axles for the rougher terrain. If you buy one that has the rubber torsion axles, they don't articulate at all on rough surfaces and you will find they ride very rough will tend to bounce all over the place, sometimes even causing them to suffer from rollovers after hitting a large bump at speed. If you buy one with a regular axle and leaf springs, the springs will not ride very smooth unless you do some work to them, or replace them for something longer, since trailer springs are meant to haul loads on pavement, not down washboard roads and rocks. The axles will also need to be upgraded. Most of these trailers will come with a 2800-3000 lb capacity axle, some with a 3500 lb axle. I would not run anything lighter than a 6K lb axle off road. Otherwise you are going to be straightening them a lot when the spindles either bend, or crack and separate from the main tube.
It can be done, but there is a reason the better built off road trailers are expensive. You can either save the money up front and spend it on labor and parts, or buy one made for the job.
