No size tag, appears to best fit a size 4/small. Shown on a size 4 mannequin and fits perfectly. See measurements below

Bust: 34"
Waist: 26" 
Length: 39" from top of strap

Floral print barkcloth fabric, sleeveless, fit and flare silhouette, back zip closure. No brand label or material information. 

Excellent condition, no flaws.