They are not the first airline to do this, and most likely won’t be the last. That said, their definition of obese means that readers with a bit of a beer belly won’t have to worry too much.
Only passengers who need more than one seat belt extender, or who can not sit in their seat with the arm rests down will have to pay.
That probably means less than half a percentage of their passengers. I’m a big guy myself, but I have no problem using the regular seatbelt, plus I can easily keep the armrests down without bothering the passenger next to me.
The extra charge will be for a second seat, and on aircraft without a spare seat, the “passenger of size” will have to wait for the next flight.
The new rules will be enforced by the gate agents, which is probably going to lead to nothing but trouble for the airline, and I suspect they will be in court with a large passenger defending this new rule pretty soon.
Their official policy can be found here.