'A dress cannot be called decent which is cut deeper than two fingers breadth under the pit of the throat; which does not cover the arms at least to the elbows; and scarcely reaches a bit beyond the knees. Furthermore, dresses of transparent materials are improper.'
Pius XII
Hmm, just a note, the website attributes that quote to the Cardinal Vicar of Pope Pius XI. Though actually, I've seen it often repeated and attributed further too.