There are several types of collars available for cats: adjustable collars, fixed size collars, safety collars with elastic, collars with breakaway buckles, collars with side release buckles etc... I offer most of these in my shop. However, now comes the important question:
How to ensure that the collar fits my cat?
To ensure the collar fits properly, you should be able to insert a finger between the collar and your cat's neck. If you have a large cat with a very fluffy neck, you should be able to insert two fingers so the collar does not rub off the fur. If you make it too loose, your cat may entangle its chin or paws in it.
Does my cat collar need a D-ring or bell?
If your cat never goes outside, then you do not need these. If your cat goes out even periodically, I would recommend a D-ring if you wish to hang a pet tag on it.
Does my cat collar need a bell?
Firstly, you need to ask yourself will your cat wear it? Some cats just refuse wearing one and will do anything to remove it (like my parents' ex-farm cat).
If your cat does not mind wearing it, then it can be used if you wish to warn local birds/wildlife. However, as you may know already, cats are very clever creatures and just putting a bell on its collar won't stop them hunting and catching a bird or a mouse from time to time.
"Dr Tim Harrison, from the British Trust for Ornithologists, said cats were able to reduce the effectiveness of bells through careful movements. By keeping their heads still as they stalk, the sound of the bell is reduced." (Telegraph)
Bells, however, have other uses - for you to hear where your cat is in the garden, or when it's trying to steal your cream cake or pizza when you are not watching....or at 3 am just when you are sleeping sweetly and your cat decided to have run around your bed... So, it's your decision.