It all depends on what you are purchasing. For their pants and skirts you can use a measuring tape and measure yourself to see what size you would need to order from them; but the lounge pants from the PINK! collection you would need to get a size smaller than you normally get. (For example I always order sweatpants in a medium so that they are baggy on me, but I have to order small from VS or the sweatpants fall off of me).
The safest thing for you to do until you are sure of what size you'll need is to go to a Victoria's Secret and try on the clothes that they have there. Once you try on clothes then you'll be 100% sure of what size you are.