The length of the trailer does not matter as much as the Weight of the trailer.
The main thing that you need to watch is the weight of the trailer. According to the owners manual for your Expedition you have a GCWR (Gross Combined Weight Rating = Expedition + All persons and cargo + trailer) of 15,000# which would allow you to tow a trailer up to a max of 8,900#.
The first thing you would need to do is find out the weight of the trailer ( Ask your sales person they will be able to give you this info), Next you would need to take you Expedition down to a Truck scale ( found at most truck stops) and have all normal family members and cargo inside and get the Ex weighed.
Then take the 15,000# GCWR and subtract the weight of the Ex = The actual weight you can safley tow.
Example 15,000-6,100 = 8,900#
For the setup you are looking at you would HAVE to have a WEIGHT DISTRIBUTION HITCH ( Preferably with built in sway control) AND ALSO AN ELECTRONIC BRAKE CONTROLLER( there is a plug under the dash for conecting a brake controller, it is located next to the diagnostic plug on the drivers side).
Hope this helps and if you have any other questions feel free to ask!