Size has always been my main concern with the 6th gen. I saw an ATS coupe at the dealership and boy did it seem small. I'm 6'2" / 290 and I felt like I towered over it just standing next to do. I did not ask to sit in it. I did set in a new Mustang, the seats were comfy, lots of leg room in the drivers seat, but I didn't have much headroom, and it felt kind of small side to side.
In my 2010 SS w/ the sunroof, I would have to sit in it with my head tilted to the side with my helmet on. It kinda sucked... a lot. I think I'd have the same problem with the new Mustang. New Camaro?
