1) have you considered the Chevy Colorado? It's available with a diesel and I've heard good things about it, although my impression was that it is still huge.
2) I am 5'10 330lbs, on my way down from ~400. I found the seat bottom a tad stiff for the first few weeks of ownership, but as with other Benzes I've had the seat now fits me perfectly. I can and do drive several hundred miles at a stretch and feel fine. I have a collapsed disk in my back and it generally hurts enough that I usually walk with a cane. YMMV
Th3) as seems to be the case with men my size, my wife is tiny. She insists she is 5' tall, but she's a tad shorter. She weighs about 110 soaking wet, and she has both a bad back and bad knees. Although she insists on picking up and carrying things 90% of her weight. She thought she'd have a problem with the bolsters when we ordered it, and we had made plans for a foam insert and a seat cover, but after a few days, she changed her mind. Besides, you need side bolsters when I drive a vehicle this tall, trust me.
4) Keep in mind both the height and angle adjustments, which make a big difference with the comfort seats. I jacked mine up so that I sit straight up with my feet planted flat on the floor- most of my driving is highway and I use cruise control.
5) in case it's not obvious, I am a Mercedes zealot. I love their cars, I put up with their foibles, and I have become used to cars that don't have adjustable steering. Besides my ML (tilt adjustment), and my Isuzu box truck (tilt and telescope) my previous cars had totally fixed steering wheels.
I like my seats firm, I sit bolt upright, and a bunch of other things. My biggest complaint about the Metris's driving position is the small size of the steering wheel. It's a complaint I have had perpetually because I learned to drive on old Benzes.
So while I am happily giving you my opinion on comfort stuff, as are others, what really matters is whether you like it or not.
But... Benz Uber Alles.