I think it's also worth mentioning that a dual-axle trailer will have a lot beter road manners than one with only a single axle. I used to tow a 21' dual-axle TT on some very unforgiving mountain roads and never had issues with sway, even during emergency stops. It helped that I was running an anti-sway/weight distribution hitch, of course.
Also remember that overall length is less inportant than how weight is distributed over the trailer axle(s).