Hahaha - I loved American girl! I had Samantha and Felicity and always wanted to dress like them. I've done two of Samantha's outfits using the real dresses, since the costumes ran big and they way they are made they can fit an adult.
The Christmas dress is a size 12:
I just added the bow and petticoat. The length was a about right although the sleeves were just about an inch short.
However, Felicity's dresses are more form-fitting and aren't made for an adult with a chest, so I used a Simplicity pattern and modified it to make her blue XMas outfit: