Length is not as important as weight, and frontal area. A Casita is supposed to tow easily, but a popup will tow even more easily. A popup is generally lighter than a Casita, and you don't have to make the bed every night. From what I have seen, many short hard-side trailers have the dinette convert to the only bed. I don't like that concept, at all.
2017.5 Lance 1995
2017 F150 EcoBoost, Max Tow
Most camping off-road