In the US. it goes by the National Institutes of Health (NIH) guidelines, which are BMI of 35 with a weight-related comorbidity or a BMI of 40 or higher regardless of comorbidities. Some insurance companies try to stipulate otherwise (for example, some say you have to have a BMI of 50 of greater for the DS), but they can usually be beaten by appeals since their guidelines don't follow the national standard.
Highest known weight: 324
Weight on morning of DS surgery: 308.5
Got down to about 185 before pregnancy;
Benjamin David born March 24, 2012, 7 pounds 11 ounces