A trailer to carry your camping gear, or a trailer to live in?
As a starting point, Smart says "no towing" but you can buy a hitch and people do tow up to 500-600 pound low profile trailers.
For carrying stuff, I would be looking at trailers sold through motorcycle dealers, although most motorcycles towing have considerably more excess power and cooling capacity than the Smart.
Fold something to sleep in, some motorcycle dealers also handle folding tent trailers, the smallest being around 400-600 pounds. I don't think you will find a travel trailer, not even a collapsible, that has facilities like kitchen and bathroom, and fold down to the frontal area of a Smart, much under 1000 pounds. The small retro TTs with just bed and maybe exterior kitchen I've been seeing have been in the 800-1200 pound range and 6 to 8 feet tall, 4-7 feet wide. Those would be the Little Guy, the 4-wide is targeted to Smart towing.
Researched this a lot looking for something to tow with my Honda Fit. Ended up buying a used one-ton van, changed the question.
Recently introduced MyPod is even smaller and more streamlined, but I have not yet seen it. Online it looks like a sleeping space only. Trailers like this are even manufactured for bicyclists to tow.
When my wife and I car camped, we managed without a trailer. Tent, kitchen, other camping gear fit into our two-suitcase sizec trunk. Don't know how carrying space of Smart compares..