A lot of yackity-yack going on here :) ;) ( hey I mean it in a good way ! )....I'll throw in my two cents worth based on some real world experience. I tow my Funfinder 189FDS with my 2006 Frontier, 4.0L V6 crewcab 6-speed manual transmission.
My funfinder weighs right at 4000 pounds, ready to roll, as checked on the CAT scale. The trailer is a 7' wide model, so it's a little narrower than some, so that very slightly helps in the aero part of it.
Have pulled close to 30K miles now, from gulf coast sea level to the high passes of Colorado ( multiple times.... I go to Colorado every summer ).
I consider this a good match. Hitched with a blue ox swaypro, adjusted to give 100 percent FALR, the rig tows well in all weather conditions I've been in.
My truck has a so-called 6100 pound tow capacity ( 6300 pounds for 2WD models, 6100 for 4WD ). I consider my combination to be all I'd want to do with this truck. I'm within all my "ratings", although I'm at 95 percent of my truck's GVWR. That tells me I would not want to "go bigger".
I've averaged 11 mpg overall in my towing mileage. The truck gets 19 or so average non-towing.
PS, I have driven Frontier's with automatic trans. I have not towed with one that has an automatic, but I am fairly certain I would NOT like it for towing. I prefer a manual gearbox in this type of truck for pulling. This is likely just personal preference though ??