Right guys, listen up. In this instance size really does matter.
Cock rings are used to help get an erection and to maintain it for longer, they can even aid in confidence boosting and can also a visual treat. A cock ring can make your cock harder by restricting the flow of blood out of your penis and this can make it harder, they can even help you to keep your cock hard for longer than usual.
It's the fact that they are designed to restrict blood flow that makes correct sizing of paramount importance. If it is too small then it will cut off blood flow which isn't good for your penis and if you get one that's too big then it won't do its job properly as it won't restrict blood flow out of the penis.