The width is proportional to the length so as to be able to harness the same proportions used in the bicycle in a combination of the two that can be no more than the Golden Mean of 1 x 1.6
Tail end width of triangle of footprint of wheel bases is the ratio of 1 whereas the length of the two wheel bases combined must not exceed 1.6
Optimal measurements are made exactly at that width by length ratio, but doing this for some time I find that there is a degree of length that extends further in these other commercial concepts, which if made with a side attached hitch will become one of the most awkward loads to carry at a higher speed to control
If your design is meant to haul ass and carry a mass, it has to use a pivotal joint that allows for the pressure to be supported by the trailer and apply only a degree to the hitch, in order to be able to be pulled and to turn without any strange buckling of the two on a leaning turn
If made wrong it will tend to jack knife on turns or apply more weight than can be controlled by the rider
