Looking over the MANY trailers in that rabbit hole, one question comes to mind is how dirty does everything get in the storage compartments. Many are using things like truck tool boxes and I would think the dust kicked up by the tow vehicle would just coat everything. Its hard to keep things clean in the sealed toppered bed of a pickup and its not in the direct dust cloud.
Dirt is ALWAYS an issue...I look at the amount of dust inside my truck bet covered with Softopper each trip and cringe.
The higher end trailers have weatherstipped compartments to protect the stuff, and the smaller Chaser AT style trailers have a seal around the top where the lid closes. I would still pack my sensitives (dry goods, etc.) in Ziplocs for storage on a run inside a trailer box.
Danny...back to one of your original questions...how big of a format.
5 x10 and 6 x10 seem to be the most popular, with the next question in my mind being what type of hitch and tongue length for the style of wheeling/overlanding you are going to tackle.
I like the smaller AT Chaser style boxes, with a fairly short tongue/hitch configuration for more responsive steering/turning off road. But that also comes into play with making sure the trailer doesn't jack knife into the body of your rig.
Another consideration is power...do you want to store your fridge on board? Solar vs dry cell? LED cabinet and awning lights are neat too...and a 7 pin connector to charge while you drive, as well as electric brakes so you exit the La Sals without smokin' hubs...the LIST goes on...and on...and on.