Measure around your waist and your hip area, and determine which of the two is wider. We call this your circumference.
The hip area is about 7 inches, or 18 centimeters, below the waist. Whichever has your greater measurement, your waist or your hips, determines the correct size for you. No other measurement is required.
Here are the guidelines:
Small is for a circumference up to 38 inches / 97 centimeters.
Medium is for a circumference up to 43 inches / 110 centimeters.
Large is for a circumference to 47 inches / 120 centimeters.
X-Large is a circumference up to 54 inches / 137 centimeters.
A larger size is usually better than a smaller size, unless you are very slim.
Note: The Pantsuit fits larger than the dress. If you wear a medium Infinite Dress, the small pantsuit is probably your best fit.