First, measure your bust and your hips. Which ever measurement is bigger that is the one you will use. Now add 1/2 of that number to your measurement and that is your width measurement for the dress.
Example: If your hip measurement was the largest that's the one to use. Let's say your hip measured 34". Add 1/2 of that measurement which is 17".
34" + 17" = 51" So 51" would be your width measurement for the dress.