Payload is ALWAYS ALWAYS the limiting factor for 1/2 ton SUV.
Short answer - for most 1/2 ton SUV and family of 4, a FULLY LOADED TT of 6500lbs almost always will stay within all limits. Subtract 1000-1200lbs of stuff water and propane means looking at TT around 5300-5500 dry. The Navigator L / Expy EL wheelbase is long enough for most trailers in this weight category. A trailer too long will more than likely be too heavy as well.
I have an 07 Expy EL and have been very satisfied with my towing experience. We usually have 6 of us in the SUV while towing the TT. The rear seat in the Expy EL/Nav L is as comfortable as the second row with similar seating height and legroom. It is the best 3rd row of ANY SUV BAR NONE. The rest of the time when not towing I am very glad to have the SUV instead of a truck. We do use it to it's full ability from hauling 8 people and overnight bags to Vegas in comfort, hauling family friends or grandparents, putting down the seats and hauling 4x8 drywall, or even a full length reclining sofa! It has met all our needs perfectly!
Long answer :
Like I said above, payload is ALWAYS the limiting factor for any 1/2ton SUV or truck. The first thing to look at for ANY potential tow vehicle is the Tire and loading information sticker. It will tell you explicitly "The maximum weight of all cargo and passengers not to exceed XXXX" That is how much people, dogs, bikes, stuff, trailer tongue THAT SPECIFIC vehicle as optioned as it left the factory can carry before reaching it's GVWR. This is not a "base" number or "brochure" number. It must be legally accurate for that vehicle within a few pounds. I will use my Expy EL 4x4. As equipped with leather, sunroof and rear AC, I have 1672lbs of payload. This is more than most 1/2 ton SUV and many 1/2 trucks. You may be surprised how low some 1500/150 series trucks payload is and will find many in the 1300-1500lbs range.
Next subtract everything you are carrying in the SUV. In my case 3 adults plus 3 kids in car seats plus a couple bags of stuff comes in at around 800lbs 1672-800=872 available for tongue weight. More people and or bigger kids will weight more and reduce that number further. Family of 4 and a dog will come in around 650-700.
Next compare this number to the maximum hitch tongue weigh rating. In my case it is 875lbs so I can use all my available payload. If it was just me I would have 1672-180 = 1492 available, but since the max hitch tongue weight is 875lbs I can't use all my payload for tongue weight. Typical SUV payload of 1500 minus a family of 4 and a little bit of stuff yields about 850lbs for tongue weight.
I would add another number that is important is the GRAWR. It comes in to play more in the truck world. It is around 4500 on the Expy EL, and is suspension limited (if you add up the tire capacity they are higher) As long as you stay within payload, you will stay within the axle rating. If you exceed payload rating, you will quickly exceed the axle rating as well! This is not always true on trucks.
Now a word about tow ratings and tongue weights. For 1/2 tons , every example I have seen manufactures state their maximum tongue weight as 10% of the maximum tow rating which can be misleading. For example, the 2015 Expy EL is 920/9200, Nav L is 850/8500 or 830/8300 for 4x4. They do not use different hitches, in Ford's case they are determining the Max tow number using curb weight and GCWR, then apply 10% to get the tongue weight. Kind of misleading.
Problem is while boats and flat deck trailers are 10% tongue weight, TTs are 12-14% say average 13%. SOOO if we pick a TT that is the max tow rating of 8500lbs for a NAV L, that tongue will likely weigh 1100lbs or more!! That is exceeding the hitch rating by A LOT and will likely exceeding what is available for payload (872lbs in my case). Where as if I was towing an 8500lb boat I would be fine! Not all trailers are created equal!
So lets find an appropriate TT. Take typical 850lbs available tongue weight. 850 / 0.13 = 6538lbs of fully loaded ready to roll TT. Most of us on here figure a family of 4 stuff, food, water, propane, batteries, will come in around 1000-1200lbs (heck my water tank alone is 400lbs!) So if you subtract that number from 6500 you get 5300-5500lbs dry weight. I have run through this exercise with many on here with many different SUVs and 1/2ton trucks, and number of people in the family, this is the most typical answer. The Expy EL is actually at the high end, which is part of the reason I chose it because of my large family size. The other extreme is something like a Sequoia, while a great vehicle, has a payload around 1250. Put 6-7 people in it, and you are done, almost nothing left for tongue weight. Even 4 and a dog at 700 only leaves 550lbs translating to a 4200lb loaded trailer or 3100 dry.
Regarding length, most any trailer 5300-5500lbs dry will not be too long for a Nav L, longer will usually be too heavy anyway.
My TT is 4100 unloaded and about 5100 ready to roll. I use a Reese dual cam weight distribution with built in sway control which works very well. The Expy EL carries the 6 of us in comfort and pulls the trailer nicely with a little margin in payload. The 5.4 can tow at the CA speed limit of 55mph easily and can maintain up a 6% grade as long as I downshift and let the engine run in the power band. I would LOVE an Ecoboost but don't want more payments.
I hope this has been helpful, let me know if you have additional questions! Good luck in your search!
