I don't care if your BMI is perfect, if you are 6'6" you are going to spill over.
You know, I kind of like this idea. They could scale their seating based on size/price too. So a larger passenger would pay more but ultimately be more comfortable. A smaller one would have less room but be paying less. My husband is 6'4" and slim, but never has enough room for his legs so I think a guarantee of legroom would be appealing to him for an extra $20-$30 bucks.
Then again, there is such a thing as first class. Maybe people of a certain size and over should be required to fly first class.
The problem is that first class includes a bunch of other stuff along with more room and is priced accordingly.
It would be nice if they would allow you to pick a larger seat and not have to pay for the lounge, free alcoholic drinks or fancy meals.