Island Planet Sleeveless Black Floral Maxi Dress With Side Slits Smocked

There is no size tag. Please read all details. Women's long maxi dress by Island Planet has adjustable straps and smocked chest area. It has side slits up each side and is made of 100% rayon. It measures 13 inches from underarm to underarm laying flat and 44 inches from underarm to bottom hem and the slits are 12 inches long. I think it would be a medium to large?