In general terms you can tow 80%-90% of the max tow rating. That's a rough estimate to allow for passengers and gear in the tow vehicle, weight of the hitch and a bit of safety margin. I'd use the 80% figure if towing in the Western US mountains and 90% elsewhere.
The tow rating is based on a tow vehicle that is empty except for fuel and a 154 lb driver. Any additional weight in the tow vehicle has to be subtracted from the max tow rating. And I personally would not want to drive a Durango at its max capacity either - the performance would likely be mediocre.
As for length, it is probably not critical. Most any trailer wheel base (hitch to trailer axle) is going to be longer than the Durango wheel base and the amount of extra is not a big factor.