Use a string or measuring tape and loosley measure around dogs neck.
Take a measurement a few inches down from his head and then add an inch for very small dogs or two inches for medium and large dogs.
Allow sufficient room to slip two fingers between the collar and the dogs neck.
Small size :Length 20" Width 14" Height 10" The Neck Circumference is suit for 6.7"-16 ".Suitable for Yorkshies,Puppies,cats,Chihuahua,Pug,Papillon etc.
Large size :Length 24" Width 17" Height 12.5".The Neck Circumference is suit for 16"-21 ".Suitable for German Shepherd,Golden Retriever,Labrador,Large Kelpie,English Bulldog,Doberman etc.
XLarge size :Length 30" Width 21" Height 15".The Neck Circumference is suit for 21"-26 ".Suitable for St Bernard,Great Danes,Irish Wolfhounds,Dogue De Bordeaux etc.
For inbetween sizes, size up to next size, or make a custom size based on your dogs size. Note: This chart is just an estimate.Measure your dogs neck and feel free to adjust.For fluffy dogs, you may need to add an inch or 2.