R&M Richard's Formal Dress Size tag is missing. Pit-pit 16.5"

Formal Black Dress Prom/Cocktail Shoulder Straps Satin Slip with tule 9" side zipper with hook & eye closure padded pleated top. This dress is missing a size tag but has no sign of wear. Approximate measurements flat pit-pit 16.5" waist 14" length of dress from shoulder strap to hem 40" Approximate Size M