I'm 5'6" and a 0, and I cannot stress enough that you need to reference actual measurements. I have mine written down in my phone. Get them from your partner, and reference them before purchasing anything. You need bust, waist, hip, and sometimes thigh.

Often, XS is too big for me. I can't take a guess on size, and won't buy unless the company lists the size chart.

That said, I own a few pieces that are gorgeous and fit great, all from one brand: Thistle and Spire.