P6 Driver
The "obese seats" will, for any given aircraft type and configuration, be those seats which are the furthest from the emergency exits, in other words halfway between any two exits.
There would be one row designated as such at each such halfway point; thus in an A320, for example there would be 2 "obese rows" totalling 12 seats. This number could be increased by designating the next rows forward or back, but only if they are not filled already with normal people.
People who can only squeeeze through a Type 1 exit would be permitted to use any designated "obese seat row" where the next exit, either fore or aft, is a proper, wide door (Type 1?). Again using the A320, this would be each of the two designated "obese rows".
If the superfattie were to disregard instructions, try to use an overwing exit and get stuck, it wouldn't matter because he or she would be at the back of the line in any event. The cabin attendant, if there, must make sure he or she exits before the superfattie, first having advised him or her politely to proceed to the front or rear door as appropriate. (That's advise politely, not proceed politely, which hopefully the superfattie would do, but it's not obligatory.)