At 6'6" and about 125 kg I have had problems with leg room on a few occassions. I always ask for an exit row and often get it, but on one occassion I missed out and the room allowed wasn't enuf to allow me to get my butt on the seat before my knees hit the seat in front. The simple solution was to stand. Eventually the nice FA came to ask me to sit as they wished to depart. I pointed out the problem, being particular to point out that I could not actually SIT, and within moments I was seated in Business.
As someone said, the airline is selling a product, transport to another location, and since I have paid, they now have a contractual obligation to provide the service. Don't get confused with the thought that you are buying a seat, you are buying transport and they fact that the aircraft is poorly laid out is not your problem. Make it be the airlines problem.
And I would not be backward about pulling out the discrimination card either.