I am in the small. I got the xs on like a straightjacket in the store and laughed and then started looking online for the S. I don't think I'm particularly tiny but the small fits in a snug, tailored way. I find I am bewteen xs and s in Banana Republic knit tops. Don't wear Theory or AT, I'm afraid.
EDIT: The closest approximation I could say is this fits like a Guess small. Barely, but M would be too big for sure on me. Hope that helps.