How small are you wiling to go, what compromises will you make in living space and fittings for small size.
I've met a number of couples traveling or camping in 13-foot "egg" trailers (Casita and Scamp) and some in A-frame folders at 10-12 feet box size. In the first case, you have to make a choice of bathroom vs permanent bed, in the A-frame the bath is rarely available, you trade off bed plus seating vs two seating areas, one becomes a bed. Refrigerators ar 3-4 cu ft and usually don't have freezers, the cooking surface will be two burner.
Next size up in molded trailers is 16-17 feet. You can get a bathroom (wet bath, shower is in the toilet space) and have the four seat dinette permanently configured as a bed, with another dinette for two. This size also lets you sacrifice the bathroom for sofa seating and/or bunk beds.
At either length these egg trailers are small in all dimensions: car width (6' 6" or less) and less than 8 feet tall, which helps keeps frontal area down for towing by cars with frontal area limits.
Next size up, 18-20 foot trailers with "15" "16" or "17" in the model numbers, conventional box construction, either wood frame or laminated panels, a variety of floor plans, some with wet bath, some with separate shower space, some with seating in addition to dinette/bed, at least one (Lance 1575) having a slideout. Three burner stovetops, and sometimes ranges with ovens, and 6 cu ft fridges become the more typical. Widths might be about 7 feet (R-Pod?) but more typically 7-1/2 or 8 wide and 9-10 feet tall.
Want a walk-around queen bed and a dinette? Length will be 20-23 feet, model number containing "19" "20" or "21" with fittings otherwise much like the 18-19 footer.
Go to some dealers, or better, RV shows, look at the small trailers, figure out what space works for the two of you. Nobody else can tell you what size fits you, and if there are two of you, it has to fit both. My wife needed about 10 feet more RV than I would have liked to have, and since she died it is too much for me, I rattled around in the space for a couple of years and now haven't used it for 18 months, the empty space is uncomfortable. Other people would find it too small.
Tom Test
Itasca Spirit 29B