It is a dress and a jacket, and my best guess would be from under arm to under arm laying flat 23 inches, and I am guessing it could b a little bigger I just don't want to tell you wrong but I can't find my tape measure and so I had to guess, sorry hope that helps, it is a size 12 and it's pretty true to the size of it, and thank you for taking a interest in it, it is a nice dress.
yes, it was from underarm to underarm, and the dress doesn't have much if any slack so make sure it will fit you before you place a bid. If your worried it might not then I wouldn't bid, it is true to it's size.