IMO, Free People pretty much runs about one size small for misses. I think probabaly for women shaped like teenage twigs, they ought to fit TTS.
I literally just returned from buying a Free People dress at Nordies, and the SA told me to buy a size M and I normally wear a size 4. I have boobs, and Free People isn't the best cut for gils with boobs. But FP is one of my most favorite brands. Ther clothes are so cute and fun to wear. But I typically buy one size up.