This is another towing question with no cut & dried answer. Weight is the primary factor, but at some point length begins to gain importance as well. The length factor is trailer wheelbase vs tow vehicle wheel base and it comes into play when side winds get stronger (including passing trucks). Tow vehicle suspension (stiff vs soft) is also a factor.
The trailer "wheelbase" is the distance between the hitch pin/coupler and the axle. For a dual axle trailer, use the midpoint between the axles as the logical axle position. It seems evident that more tow vehicle wheelbase is better. I've seen suggestions that a 110" wheelbase on the tow vehicle is suitable for a max 20 ft trailer, but I can only guess what trailer wheelbase that implies. Probably something like 150"-170"? Trailer axles are typically 2/3-3/4 of the way to the rear.
In his book ?How to Select, Inspect, and Buy an RV? by JD Gallant (of the RV Consumer Group.", Gallant recommends that a ?20 foot trailer needs a tow vehicle with a wheel base of 110 inches, and for each additional foot of trailer length, you need an additional 4 inches of wheel base?. He makes it sound definitive, but he is just one guy with an opinion - RV Consumer Group does not do any testing and has no particular engineering expertise. In other words, just another opinion (as mine is as well).
Supposedly Gallant analyzed trailer accident records to come up with this recommendation, but accident records do not routinely record wheelbase, either tow vehicle or trailer, nor are the trailer lengths more than an approximation. Further, the cause of an accident may or may not be related to trailer length vs tow vehicle - there are many possible factors and more than one can play a role.
Per JD Gallant:
Trailer length_____TV Wheelbase required (based on rule of thumb)
-------------- ---------------------
___20'___________________110"
___21'___________________114"
___22'___________________118"
___23'___________________122"
___24'___________________126"
___25'___________________130"
___26'___________________134"
___27'___________________138"
___28'___________________142"
___29'___________________146"
___30'___________________150"
___31'___________________154"
___32'___________________158"
___33'___________________162"
___34'___________________166"
___35'___________________170"
___36'___________________174"
___37'___________________178"
___38'___________________182"
___39'___________________186"
___40'___________________190"