The Fi collar is lightweight and can comfortably fit dogs 10lbs or more, with neck circumferences' of 11.5" and up. While we don't have an extra small size developed at the moment, we hope to have one at some point in the future!
If you're interested in an extra small Fi collar for your dog, please email support@tryfi.com, and we will be sure to pass your interest along to our team for future improvements!