What size should I get?

I’m so confused by the sizes of this jacket. I am 5’3 and I want a jacket thats slightly below the knees. I’m very skinny. Help?

http://www.beautyboutique.com/cgi-bin/beauty/prod/057369/item_detail.html?afsrc=1&cm_mmc=PaidSearch-GooglePLAFreeShip_057369110SP&media=3GOOG44A&gclid=Cj0KEQiA-uWjBRChu9ie05m5ipgBEiQAuHIuKzVZ_aXZGGBnjreOfDdAER00pbHK0TCiHE2UQnJ3FIAaAiac8P8HAQ#pr-header-back-to-top