Everyone is looking for a black and white answer and there simply is not one. If you are a big person you can follow the COS policy and get the free extra seat, nobody is going to debate whether you were "big enough"
Nobody is going to ask you for proof you need the wheelchair or if you really need to preboard because of your sprained ankle causing a slight limp.
The entire SWA system is based on people self reporting and I suspect that will eventually be it's downfall