Thank you all for wonderful advice and wisdom!! @Kirk W great formula! Did not know this...my Wheelbase is 145 inches which would put me at about 28ft (I assume this is total trailer, not living area?). In looking at this further, I was able to message with Ford today and ask them - they said 12,300 towing capacity, but went to the grid online (thank you @Tulecreeper) and think she was incorrect, and it is actually 11,000 (will confirm w dealership tomorrow). This whole process has opened my eyes and I think the challenge will be with the tongue weight, as well as possibly towing. I am seeing tongue weights between 700-950 lbs for the trailers we are looking at.
I used the calculator on Ford's website with my truck specifics, and it lets you edit / configure weight of passengers, cargo, and accessories and then outputs a Max Tongue weight and Max Trailer Rating. I went ahead and added 450 lbs for passengers, 50 lbs for cargo, and 50 lbs for accessories and the outputs are:
Max. Tongue Load - 782 lbs
Max. Trailer Rating - 7,820 lbs
Here are a couple of examples of trailers we were previously investigating...Even though the 2 examples below are over the max trailer rating that the Ford calculator output, I do not think we would add more than 1000lbs for us 2 people of cargo in the trailer.
Trailer 1 - 32.3 Ft. Exterior length
Hitch Weight - 690 lbs
UVW - 6,845 lbs
GVWR - 8,495 lbs
Trailer 2 - 28.6 Ft Exterior length
HITCH WEIGHT 680 lb.
UVW - 5,906 lb.
CCC - 3,574 lb.
If I were to use the Ford calculator as gospel, both of these would be at the limits and pushing it bit I am not sure if I am over-thinking this. @fpmtngal good use case there, and I am thinking the data from above + your friend who was happy with 25ft box trailer and the Hybrid F150, it may make more sense to step down to a ~25Ft exterior length. For background, yes we would use it in southeast predominantly along flat areas and for weekend or week-long trips. Thank you for any other advice, much appreciated!