I have a 21 footer. I have a toolbox at the front, and my Challenger fits nicely. It's always better to get one a little bigger than you need, because you never know when you'll run across that next great deal, and it may be a bigger car like a Charger or Road Runner.
If you want the absolute minimum, measure from the front bumper to the back of the rear tire. Add the depth of the toolbox that you want to attach, then add at least one more foot for safety margin and jockying room. But remember that your car will stick out past the end of the trailer.