How big is too fat

To answer your heading question directly, any obesity is too fat: it is unhealthy and the more overweight you are the more dangerous your condition.

I realise some people are morbidly obese and struggle to reduce their weight to a healthy level, so need professional help; but there is also a strange and very dark fetish for becoming crippled by obesity. How to deal with that, I have no idea. It's definitely a medical problem.