I ordered it in a size large, and find it fits the way I like it. I wanted it to be slightly roomy, not huge, and that's what it is. I normally wear a size 12, and I'm tall with a fairly full bust.
I liked the gown so much, I ordered a second one.
I think it runs true to size (hopefully it's still manufactured and sized the way it was when I bought it a month or more ago). Good luck.