Size 20-30 cm is for very small dogs as chichuahuas, maltese, pinchers, ...
Size 30-40 cm is for small dogs as beagles, schnauzer, ...
Size 40-50 cm is for medium dogs as border collie, tollers, a, ...
Size 50-60 cm is for bigs dogs as labrador, leonberger, bernese mountaindogs , ...
Size 60-70cm is for giants breeds as Cane Corso, boerboel, newfoundlander, ...
Maintenance
To clean the material, it is best to use lukewarm water and NO soap. Soap can discolor your hardware if you didn't choose stainless steel. It is best to wash them with a soft sponge or your hands.
If you didn't choose stainless steel, try to keep your hardware out of the water and pat it as dry as possible with a towel. The non-stainless steel hardware can of course rust if you do not dry them well after swimming, washing, ....
It is best to dry your items with a towel and do NOT just let them air dry. It is necessary to remove most of the water from your collar first to prevent discoloration and damage.