The hitch shops are asking the right questions. The height, width etc. of a trailer does not matter to the hitch selection. What does matter is the tongue weight and how much weight you need transferred back to the front of the tow vehicle. What type of sway control you want can enter into the selection later.
For example, the Reese "Dual Cam" is an excellent system but requires a minimum of 400lbs of tongue weight to function correctly. It would probably not be the first choice for a 13ft lightweight trailer. There are other WD hitches that have sway control built in that would work better in that case.
In many cases, a standard round bar WD hitch and friction add on sway control is all that is needed. In other cases, you would want a hitch that had better ground clearance and much more sway control so you would select a different style or brand.
In all these cases, the shape of the trailer has no or little bearing on which hitch to select. It is all about size (which usually relates to tongue weight), floor plan and storage capability ( which also relates to tongue weight), and planned use.
TW and bar strength are the main part of hitch selection but really are just a part of the sway control formula. There are many other factors that enter into whether a trailer will sway or not. The sway control is there to help when Mr. Murphy steps in your path - not to fix a bad towing trailer.
Hope this helps a bit.
Barney