Width would be more of an aerodynamic issue if you didn't haul a TC. I also have an 8.5x20 enclosed Interstate trailer. I researched for a while and found 18' is pretty much the minimum length you want for a car trailer unless you have a really small car. If you plan to carry some items in the trailer along with the car, you need more length. I am sensitive to length since we pay ferry charges based on it. By staying with a 20' trailer, I can keep my overall length under 50' instead of paying the next increment higher toll. We also camp where there are some tight turns defined by trees and rocks - My current setup is very manageable even the older state parks. If my toy collection continues to grow, I will not go longer, but taller. I keep eyeing the 20' stacker models with internal lifts as my DW rolls her eyes...
If you plan on hauling a car, consider getting a trailer with a side escape door so you do not need to climb out the window when you load the car. I went with 5200 lb torsion axles verses solid ones with leaf springs and find the ride to be much better for anything you plan to haul. 3500 lb axles are too small to handle the weight of the trailer and good sized vehicle.
If your trailer will also be a shop or man cave when not hauling vehicles, price out the electrical and air conditioning options verses installing them yourself. Some people like windows, I prefer the privacy and security of obscurity by going without (you know that big flat screen will ideally be mounted right where you put that window).