She is right between the xxs and xs, but the xxs is tiny. This doesn't have the xxs on it for some reason, but that will give the head strap and chin strap size.
If you have picture I can help with that too, but the xxs is really tiny.
Eyewear Typical Size Dog Head Strap and Frame Chin Strap
XS 1 - 10 lbs 5 - 13 inches 3 - 7 in
.5 - 5 kgs 13 - 33 cm 8 - 17 cm
Breed example: Chihuahua, Pom
Small 9 - 25 lbs 12 - 20 inches 4 - 8 inches
4 - 12 kgs 30 - 50 cm 10 - 20 cm
Breed example: Beagle, Westie
Medium 20 - 60 lbs 15 - 25 inches 5 - 11 inches
9 - 27 kgs 38 - 63 cm 13 - 28 cm
Breed example: Dalmation, Border Collie
Large 50 - 100 lbs 18 - 30 inches 6 - 15 inches
22 - 45 kgs 46 - 76 cm 15 - 38 cm
Breed example: German Shepherd, Labrador Retriever
XL 100+ lbs 22 - 45 inches 11 - 26 inches
45+ kgs 56 - 114 cm 28 - 66 cm
Breed example: Large Rottweiler, St. Bernard |