“A mortal sin of scandal is committed by women who go about with their bosom immodestly exposed, or who expose their limbs improperly.” St Alphonsus Ligouri

Particularly about the limbs? Though it would be interesting to know what he ment on the bosom. Because attire in his time was very different to modern times.

About the bosom comment: St. Alphons knew history very well. There were very bad costums at different times in Italy (Renaissance Florence for example). So you probably understood him right.

I am actually not sure what he means as I am not familiar with the attire in that time. Nowdays most ladies wear tops that show the skin between the bosom and even much of the bosom itself. I also don't understand his limbs comment. Does his mean arms and legs should be fully covered to wrists/ankles? Or certain kinds of 'cuts' should be avoided, etc a slit leg dress.

Regarding the chest and limbs, women's dress was considered to fall within "Marylike modesty" as long as the top did not fall under two finger widths below the base of the neck, the sleeves reached the elbows, and the skirt covered the knees entirely when sitting. This was in the mid 20th century when liberalism and feminism were already widespread, so I would assume St. Alphonsus' standards would be higher (e.g., skirt covering ankles, sleeves to the wrists, nothing at all beneath the throat exposed). 
Now, St. Alphonsus speaks of the "bosom" being exposed, so I don't think two, three, or even four inches below the neck being bare would be be considered a mortal sin of scandal..as that's not really the "bosom". Probably just a lesser sin of immodesty.
 With the legs, I could see him meaning as a mortal sin of scandal the calves being entirely exposed. That would seem somewhat excessive to me, but that could also be my liberalism talking