It's a Alvina Valenta (9102) dress from her Couture Collection. It's a mermaid style gown with ivory lace overlay and a honey coloured underlay. A corset-style sweetheart bodice with a built-in bra. Button details from the top back down to the train. Also has a French bustle.
As for sizing, the dress was made and altered specifically for the measurements in the ad (or close to). There is no specific size for the dress.