Martha Stewart mediums are way too big for my Saydee, but fit good in length, so I would go with a small for a 6-7 pound dog. Is this at Petsmart? Most Petrsamrt XS are really tiny. My Minnie generally wears an XS, but I have to get her a small there. I purchased her a S in the MS line and had to take seams in it and move the straps over, but I'm not sure how their XS runs. Without the measurements, it is really hard to say. I know I'm no help,
