Zara White Elegant Dress (see info for sizing)

White, elegant (somewhat tight) dress from Zara. Down to knees (I’m 5’5) and it’s backless in the back. Also has some small black jewels embellished on shoulders. It says it’s a medium but I cannot fit in it and I’m a small, so I would not recommend this for girls with a bigger butt or broad shoulders! Also has a zipper on the side. Would recommend this for a petite girl size S ideally.