Here's what I've come to learn after being in business now for over 5-years. There is no universal size chart. Each brand measures their clothing differently. So the best way for a plus size woman to know if something is "true to her size" she must know her three basic measurements. These measurements are her bust width (measuring armpit to armpit) waist, and hips which is the widest part of the body.
At UrbanThick, we add these measurements on all of our items. Most plus size women will just purchase clothing from one store, because they are familiar with their size chart. However, if they go that route, they'll be missing out on some pretty awesome styles.
So, remember, don't worry about it being true to size, just know your measurements to get the right size for your body.