I have an odd body shape, so, if I'm buying this online....?

Wet Seal is a pretty popular company that you are purchasing from, so search for the name of the company + sizes run large. At the Gap, I am a size smaller than I would normally wear. Also, you can e-mail the company and ask if their sizes run larger or smaller than normal. If you cannot find an answer, then just buy it in a medium and wash and dry it to shrink it (if it's cotton) OR just look for a substitute at a store you an actually try it on at... maybe they sell it at the actual store.