Hi, DrJones:
I'd say how wide and long you make it has a lot to do with the pulling strength of what you will be using as a tow vehicle.
If you're looking to save gas mileage by staying within your tow vehicle's lines, you already know part of the answer.
Another point: You can pull more weight if you have brakes on your trailer. Weight is a big factor with this one. When your trailer reaches a certain weight, maybe 1500 lbs, many states require brakes.
If you have a new vehicle or plan to get one, check the owner’s manual, in the towing section and the brakes section. That will tell you what you can do without negating the vehicle warranty.
These are some starting points at how to come to your trailer size decisions. Others will chime in with their opinions.
