Gaining

How do you decide when someone has grown so fat that he/she can be considered "super-sized"?

I've heard..... as far as weight goes it's 350 for women and 400 for men...

Given height is a thing, I would say BMI matters more

I wouldn't consider myself a ssbhm until maybe 500 because I'm at 380 right now and would only consider myself a bhm but I'm also 6'4" so I'm tall also

SSBBW/SSBHM

When small fats either:
A) Use your photo as 'thinspa' on their refrigerator
always want to hang out as their physical trainer said its ideal to push food on someone else to eat, vs eating the food they normally would themselves
C) Crack jokes at your expense to appease their thinner family and friends

are key signs you're heading towards super-size territory.

Just weight alone wouldn't be all that useful, because there are differences in both height and build, which would make someone super-obese and someone else merely plump, at the same weight.

What your body can do and where it can or cannot fit, would be a better criteria.

So, I would say it's where one starts getting bigger than most things are designed for. Chairs, cars, seats, booths becoming too small to fit into comfortably (or at all). Certain body positions and movements starts becoming very difficult or impossible (tying shoelaces, walking up more than a single flight of stairs without having to stop, etc.). The threshold would be when going through narrow doorways, fitting inside small cars and behind restaurant booths is still possible, but only with great difficulties. Where every physical activity is still possible, even if only for very short periods of time. A size which starts seriously affecting what one can or cannot do, but still not as big to make most activities impossible.