I'm 5'6 and weigh 116 I actually do wear a size 3.  The heaviest I've ever been is her skinniest (135lbs) and when i did weigh that much I wore a size 9 and I had an extra 7"  hight on her so she would be a larger size than I was. Im guesing a size 13 minimum at her smallest. Of course this was years ago before all the vanity sizing bs so maybe if you count that she could pass for what  is considered a "size 9" now.