I think it is the XS like I got Rufus. I haven't really needed Rufus's, as the long line has not been needed in the new regime. Rufus is the maximum girth for the chest strap, but the fit was exact at his private parts for the other strap. Don't think he liked his-used it 3 or 4 times, but absolutely brilliant for picking up the dog one handed for stiles etc.. I thought it was the best harness we had tried, but the sizing is difficult for a short wheel bas cocker like Rufus. His was £40, so not cheap.