There are many class 1 trucks I like. But due to the weight limitations of the semi floating rear axles, I find they can rarely tow a typical trailer to rated maximum due to the tongue weight. You see this same thing in class 2 and class 3 SRW trucks once you are looking pin weights of fifth wheels - The limitation on most of these are the rear wheel and suspension capacity.
As far as length, I do believe longer trailers are more stable. However, they have more surface area to catch wind which can induce sway.
Tow vehicle length also plays into stability while towing. When we towed a popup with a Ford Bronco II verses a Mazda 2600i Cab Plus, the Mazda's longer wheel base handled it better.