You need at least 2 OR 3 sizes more than you actually wear. And yes, it streches, but not much and even so, the material looks ugly when streched and the dress becomes more like a t-shirt than a dress, it won´t cover your butt. I made a picture with my normal size hand so you get an idea, well that is size M, fine for a t-shirt, not for a dress

I love this dress it fit perfectly