Enclosed cargo trailers are nice to have. I definitely am getting one, once I make the jump to a new rig (am biding my time because I don't have much vacation time right now until next year.)
I'm also not sure on size. I'd like something big enough to toss some solar panels, maybe go with 6-7 feet at the widest so there is room for adding panels on the sides that can fold out.
One thing I would consider with a trailer -- two doors. The usual one that opens from the back (be it a ramp or lockable double doors), and a side door that has a RV handle lock, as well as some barrel locks on the inside. That way, no matter what, one can get out, but a would-be thief would have issues getting in even if they broke open the RV handle. It also allows the trailer to function as additional sleeping space in a pinch.