Southwest's (WN) rule seems pretty common-sense: if a passenger cannot fit in a seat with the armrests lowered, they are obligated to buy two seats.
This is the essence of the Rule now being proposed as an EASA AMC to Regulation Air Operations (EC 965/2012, effective 28th Oct 2012, as everyone knows), as mentioned above. It is couched in rather different terms, and focuses on a fatty's ability to use the Emergency Exit as rather more politically correct than his/her weight, girth, appearance and smell, so as not to offend the European Commission's numerous and varied sensibilities. But it amounts to the same thing. And there will be other critieria added at the Working Group stage of the process, such as those suggested by
Jarvy and
Sober Lark, of which EASA is now aware.
PS In breaking news, EASA is struggling more than usual trying to translate the draft TOR for the Working Group; it was originally drafted in Gaelic (to satisfy quota issues and practice for the day when a Gaelic-speaking Scotland joins Catatonia as an EU Member) with a title roughly translated as "Safety of the Grossly Obese and those in their Vicinity". The Spanish went for "eliminación de las tinas de manteca de cerdo", the Hungarians preferred the pithier "forgatás az elhízott", which the Poles matched, in a sense, with Slavic solidarity, choosing "strzelanie otyłych". The French interpreter said "Je m'en fou" and retired to his favourite Cologne restaurant. The Germans said "Wir urinieren auf euch alle" and went back to the bar, beating up a few Serbs on the way. The Finns stripped off and went to the sauna, followed by the Swedish and Norwegians. The Greeks, Portugese and Italians could not be woken up. The Bulgarians, Irish and Czechs could not be revived. Slovakia wanted "Nariadenie aby obéznych ľudí na zemi", which was almost accurate. The UK CAA had 8 meetings, came up with "Regulations for Controlling the Impact of Very Large People on Passenger Safety" and went back for more tea. And so on; it is likely that the Regulation will never emerge from this Black Hole of Babel, as so many others have not done.
The Tower of Babel