Sizes XS (6) and S (8) - if between sizes, size up 

Description from Ance Gria website:

The Atlein Gown in Plum. 

A sleek full length maxi dress with a scoop neckline and ruched bodice that contours the waist. The straight silhouette falls cleanly to the floor, finished with a small back slit for ease of movement. Crafted in a smooth glossy stretch duchess satin.