I don't shop for myself, so I'm not quite sure how to help you.
The problem is that women's clothing isn't standardized at all. A size 1 isn't always a size 1, sometimes it's 1 sometimes it's more like a 4. I personally blame fatties that want to feel thin.
The most important part is the fit on the shoulders and chest. Sometimes my boyfriend get's me a dress that's a little baggy on me but you can fix that with a nice belt, but I have a nice silk sash.
My boyfriend knows how big I am and can just visually determine if something will fit me. So I'm afraid you might need to physically look at whatever dress you want and try to gauge if it would fit.