How big of a TT can an Expedition safely handle?

My take is that if you stick with a trailer with a loaded tongue weight under 1000 using a good well adjusted WD hitch, you will be ok. Remember that the optimal tongue weight to minimize sway potential is 13 percent of loaded travel trailer. A trailer with 7000 gvwr will likely have a loaded hitch weight of around 900 to 1000 lbs. Therefore, the consensus of a 7000 lb or less loaded trailer seems fitting for your TV.