KT00 stretchy lace dress sz M or S? NWT

New with tags condition. Beautiful stretch black lace with beige stretch underlying. Size states Medium. Seems like a Small. Measures armpit to armpit 15" (before stretch). Front top to bottom 40" midi length?. Compare measurement before purchase. Made of 90% nylon, 10% spandex. From a smoke free and cat/dog friendly home. (5)