The mannequin I use wears about a modern size 6  comfortably. The dress is too small to zip up on the mannequin, which is why the zipper is shown open. The zipper works great and the dress is in amazing condition. It would likely fit a modern day size XS. 

Length: 47". Underarm to underarm: 15 1/2". Waist: 12". Sleeve: 11". 

Material: 100% Cotton.