35 inches (women) or 40 inches (men) -- Waist Circumference
Studies suggest that people with larger waists are more likely to develop heart disease.
Waist circumference is even more useful when used in conjunction with BMI, and The National Heart, Lung and Blood Institute has a chart where you can gauge your risk of developing obesity associated conditions, including heart disease, based on both numbers.
If you don't know your numbers, see your doctor for a checkup and blood tests. Once you know how your numbers compare to the ideal, formulate a plan to bring any out-of-range numbers back in line.