I'd go with XS. I've not tried that shirt on - it was on my spring wish list, but once I saw it in the store, I didn't even bother. It's not the same color (it's a much brighter (ickier in my opinion) color, and it also is much more blousey than that pic shows, which is what makes me think you'd want the XS. And the fabric is really, really thin. :(
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"Good taste shouldn't have to cost anything extra." - Mickey Drexler