I have some pics from the dog show of different size pups in the small sling...that was the only size we had at the show. Perhaps some of these pics will give you an idea.  You also might ask in the thread in the sale forum but I think most have either small or large. I don't recall anyone buying an extra small one.
  http://www.yorkietalk.com/forums/yor...og-slings.html 
This first one is katy-yorkie ... I am sure if you PM her she could tell you about her sling.  She has a size small and she is tiny herself.   
 
   
   
 
This is Kiche, my pup.  He is 4 pounds and is in the small sling.  I am tall.   
 
 
Hope the pics help a little bit. 
