With regards to width,
I say go as narrow as possible as it provides huge benefits.
The only negative? It detracts from usable interior space.
For us though, our (narrow) camper is a game-changer. Ours is 82" wide as I recall with a 78" interior width.
Overall height is 10'4" We scrape under trees often, and trim as needed. But if it fits, it ships
We just finished a trip that involved 70+ miles off the pavement on little more than single track Jeep trails with nasty drop-offs, and any wider would have been a serious problem.
We came head-to-head with 2 vehicles on the trip, and each required one of us to reverse more than a 1/4 mile to find a pull-out large enough to pass....
Height will always be a concern for a hard side slide-in, but when the width is similar the width of the vehicle, is is considerably easier to manage.
And more power to ya if you can get this segment rolling.
There are loads of reasons why we decided upon a custom flatbed camper for our needs.
I hope more and more people realize the huge benefits of a flatbed camper on a sturdy, comfy full-size pickup.