If gas milage is the big deal for you a 4X8 would be better.
Seriously, I know gas prices are higher but it can't be that big of a deal. I've pulled my 6 X 14 X 6.25 behind our Jeep Cherokee Trail Hawk and you don't even notice it until you get to a long 20 degree + grade. A Gladiator would yank it up hill without a problem I'm sure. Did you buy the Gladiator for it's great MPG?
You must have a monster Boxer, mine is 75 pounds full grown. Not only does it need its own bed, it needs room to walk curled around following its butt.
A 6' wide will offer you many, many more options for your floor plan, the big difference is that, at your height, you can sleep N/S or E/W and E/W saves real estate. Wider can also mean a shorter trailer!
If you get a V Nose, 6 more inches on both sides won't make much difference, if any at all. What racks up weight that drops MPG is the length carrying more weight.
(BTW, you can still have an exterior kitchen about a foot inside of a cargo trailer with a ramp door, just put a passageway past the kitchen wall to walk out on your own elevated patio RAMP DOOR.

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If you must crawl up rock ledges at Canyonland, I'd go with a 4x8 and tentage out the back and sides. For a cross country jaunt with dirt roads, forest roads, I don't think a 5 or 6 foot wide would matter much.
Google just told me a 2023 Gladiator is 73.80 inches wide, that's 1.80 more than 6 foot.
It's ideal for your trailer tires to track in the footprint of your vehicle tires, that's "ideal" mainly for off road rocky trails, deep mud, narrow passages, etc. Question to ask; How often do I need "ideal" specifications for my intended purpose? My guess is not very often in reality. Being off 6 inches is close enough.
A 6 X 10 might be better if smaller/lighter is really needed, but you'll end up missing that 2 extra feet (or 4 extra feet after you realize what you've done).
My situation is two adults, neither of us are petite, and our Boxer, it's really nice having a 6x14, a 6x12 could work but it would be tight, smaller isn't an option for us.
I also suggest you figure out what you need inside, toilet, fridge, sink, beds, seating/table and toys. (Wind and rain will make you wish you had something worked out to prepare a meal inside, it's nice cooking out but it's not always worth the effort.)
I'm 6 foot, or I was until I shrunk a bit getting older, mine is 6' 4" tall inside, that comes down adding insulation. Most are probably going 6'6" or 7' tall, that is really too tall for a 5 wide, especially if you use the roof to store anything or have solar up there. A 6x6.5 is nearly square.
If you go light, a single axel, 3500 lbs should be fine, match your trailer and vehicle tires and carry another spare. Single axel is best for off roading IMO, if you are taking a full queen size bed, apartment fridge, 100 lbs of dog food, 2 weeks of grub and half your clothes closet, go with a double axel, 7500 lbs or more. Also, get a straight axel set up, not drop axels as you can raise the unit, use air bags or heavier springs for off roading, if necessary.
I'd suggest a 6x14x6.5 V nose, then 6x12, then 6x10, I wouldn't consider a 5' wide trailer behind a full size vehicle.
