Missoni M Cocktail Dress

Missoni M Dress Green/Blue/Black Cocktail Black Sheer top Worn once, purchased in NYC at vintage shop Small/Medium - fits a larger small if that makes sense. I used fluctuate between a small and medium size and this dress would fit me, but when i have gained some weight it was not comfortable. I am now consistently a small and it fits comfortably. There is no size on the dress indicated.