The tall fat guy suit would be the same as what us tall skinny guys wear. The lycra pulls in the excess and it looks like a custom suit either way. One of our local guys is my height (6'2"), and outweighs me by 60 lbs - enough so that he can't wear a weight belt, because it just falls off of him. He wears the same suit as I, a 2XL-3XL, and it fits him perfectly.
It is tough to believe, I didn't until I actually was inside one, but this is a suit that makes custom obsolete. As long as you fit the general parameters of height and girth, the suit fits like spray paint. I've had a lot of custom suits, and none fit the way the Fusion does.
I ended up in a 2XL-3XL because of inseam. When standing on the floor in stocking feet, I have a floor-to-crotch measurement of 34" (this BTW is a measurement that amazed me, I had thought it would be close to my pants inseam of 36, but until you actually measure yourself you don't know just what it really is), and this gave me a suit leg that was too short in the M-XL size Fusion. In conversations with Tyler he passed on that newer versions may have a retailored leg (our were one of the first 200) so this may have changed. Still, I am very happy with the 2XL-3XL, and except for the size tag I'd really never have noticed what size it is.