This same topic always seems to pop up every couple of years. In the past I have offered these categorizations.
By BMI: The whole point of BMI is to normalize weight over different heights. Two people with the same approximate build but different heights will have the same BMI. BMI doesn't account for muscle vs fat, and fat itself isn't the best indicator of health, which is why BMI has a bad reputation due to doctors using it as a proxy for health. But most people are not athletes or body builders, so the difference in muscle mass and its effect on BMI is negligible for average people. So BMI is a pretty good way to quantify fatness. As far as categories, I like the breakdown of 30+ is BBW/BHM, 60+ is SSBBW/SSBHM, 90+ is USSBBW/USSBHM.
By clothing ranges as suggested by Doe_Nylie above: This is a lot more subjective, but reflects the reality of life as a (SS)BBW. If you can find clothes in generic clothing stores but have to go to the plus size section: BBW. If you have to buy your clothes online because even Torrid and Lane Bryant don't actually have your size in store: SSBBW. If it's hard to find stuff that fits even in online shops, USSBBW. This breakdown doesn't work as well for men, though, because as much as clothes shopping sucks for fat women, it sucks even more for fat men. Most department stores that have a plus size section for women have only a couple of shelves of Big and Tall clothes for men if they even have B&T section. I had to switch to online shopping for clothes many pounds ago, and I wouldn't consider myself quite SSBHM yet. (5'9", 350 lbs)