Fit#
You should be able to slide two fingers flat under the collar comfortably, but not much more. Too loose and it can slip over the head; too tight and it can rub or restrict breathing.
Check the fit regularly, especially with growing puppies, since collars can become too tight within weeks as they grow.
Material#
Quality leather softens with use and typically lasts for years with basic care, while fabric collars are lightweight and often easier to wash.
For dogs who swim often or have sensitive skin, a quick-drying or waterproof material can be more comfortable than leather, which can stay damp for longer.
ID#
Add an ID tag with your contact details so your dog can be identified quickly if they ever get lost, in addition to any microchip they may already have.
It's worth double-checking that tags are securely attached and details are current, particularly after a house move or a change of phone number.