Hey I just happen to have a 2004 F150 with a 5.4. (BTW. Chevy does NOT make a 5.4, it is a 5.3)
Any way, Were I you. I would look for a TT in the 4500 to 5000lb dry range. And loaded ready to camp in under 70000lb range. Not that some more would not be safe. I just don't think you would enjoy the tow as much. I believe you, and the TV would be much happier in those parameters.
As to length. Not really a factor. Weight will be your limiting factor, as TTs are getting lighter because of the new space age material. My 31' Apex is lighter than my previous 27' Pioneer, and it had no slide.
How to know the weight of the TT you chose? On NEW TTs there is a yellow sticker. On that sticker is the actual weight of the TT as it sits on the lot in front of you. That is the dry weight, meaning the weight BEFORE any liquids are added. And like mine. It includes the weight of the battery, and EMPTY propane tanks. The sticker will also tell you the weight of full holding tanks on another section.
It is up to you as to how much weight you add to the dry weight. We added about 600lbs. But we came from popups, and everything we have is light weight, and compact. But we carry 9 folding chairs, tables, camp stove, pots, pans, towels dishes, sheets, cups silver ware, blankets, coolers, grill, games, tarps, and more. Again I would try to keep it under 7000lbs. actually easy to do. And since you know about how much you will add 750lb. You will be good to go with a 4500 to 5000lb dry TT.
And either TV you will need a WDH. On the 150. You will need to adjust it so that the front comes back to the unhooked height, and the rear is no higher than it was unhooked. New Fords are different. But that is the instruction for that year.
Good luck, and have fun
Terry & Shay
Coachman Apex 288BH.
2013 F150 XLT Off Road
5.0, 3.73
Lazy Campers