I've posted my cargo trailer setup a couple of times.
It never really got 100% finished. The original design was to do a double layer deck that would be strong enough to load stuff above the wheels/fenders. But, I had a tough time with welding. I think I know what I was missing now, but need to practice a bit more.
I had also intended to connect the old coaster brakes to mechanical surge brakes controlled on the tongue, but really haven't found them to be that necessary as the heavier I get, the slower I go.
On occasion, I've simply put boxes or boards on the deck to get above the wheel height.
Anyway, if you are planning to occasionally carry something like a 4x8 piece of plywood flat, I'd build the trailer about 3 feet wide, and 5 or 6 feet long (which can get through most bollards), but add provisions to carry stuff overhanging a little bit with the load above the tires.
I did a straight pull-behind tongue specific to my cargo bike. I like the simplicity of that design, but might consider a regular side pull hitch. Still, I like the cargo bike for towing.
One of my next builds will be a heavy cargo trike, capable of very low gearing (and possibly pedal assist). Probably mainly for towing. 5th wheel?