black long satin prom dress with off the shoulder sleeves

bought from a prom dress store but very similar to house of cb loretta dress very luxe and elegant so flattering on the body can tighten as there’s a corset at the back tag says size 12 but the worker told me the size is just for how high the slit goes (larger size = lower slit) as the rest back of the dress can be tightened modelled on a size medium for reference bought for around $300 message for more in depth pics!