Help with dress sizes?

I would really like to buy some dresses for this summer, and also for next fall. I have no idea what size I would need. I'm not even sure I would fit into some of the dresses at Forever 21, Aeropostale, American Eagle, etc. I take an XS in shirt sizes, and they're still long on me. I have a really small bust, a 32A. I am 57" tall, really petite, and I can't even fit into a 00 at Aeropostale without having them fall off me. Do you think I would fit into any dresses at those stores (could I tighten the straps all the way), or would I have to shop somewhere else? Do you know any stores that would sell dresses in my size? I will to the stores as soon as I can to try them on, but they are far away from where I live, so it might be awhile. I was just hoping to get an idea of what I might or might not fit into. If not, I can just stick to wearing skirts.