Sizing info

For Collars:
Extra Small - fits necks 7 to 11 inches
Small - Fits necks 10 to 15 inches
Medium - fits necks 12 to 19 inches
Large - fits necks 15 to 24 inches - (this is the size of the collar in Pics)
Extra Large - fits necks 17 to 29 inches
For Harnesses:
Measure the girth (the dog's chest)
Extra Small- 9-15" with 5/8 of an inch in width
Small- 14-21" with 5/8 of an inch in width
Medium- 17-24" with 3/4 of an inch in width
Large- 22-30" with 1 inch in width

Tips for sizing Collars and Harnesses:

Remember that puppies grow quickly, so it is important to check the tightness of your dog collar, harness, or muzzle often.
Inspect and replace a collar, harness, or leash as it shows signs of wear.
A rule of thumb is to leave an allowance for two fingers to fit between the collar or harness and the dog for a comfortable fit.
Dogs, like horses getting fitted with a saddle, may hold their breath while they are getting measured, therefore expanding their chest measurement.

 If you don't have a tape measure, your can measure your dog's neck or chest with a string then measure the string with a flat ruler.