I would get the XS since it is for 17"-24"

Don't forget to factor in her hair when taking the measurement; but since the XS covers a wide size range, you will be fine. I haven't had any sizing issues, except when I measured around the puffy hair instead of against the skin!

I will also make a quick review/note:
the Portage lifejacket has a little more flotation and should be picked for dogs new to water or with less buoyancy or as just a standard lifejacket.

If your dog is an active water dog, then I would recommend the the big eddy lifejacket (BC #RUF0050) as this one is a little more streamlined if the dog is going to be wearing it for many hours/days.