I got this info from a website.
"To calculate the minimum size of a cockring, cut a strip of paper or ribbon no wider than 1/4 inch. Starting from the top of your penis at the base (where it meets your body), wrap the paper under your scrotum, bringing it around to the starting point. Mark the paper where it meets. Measure that length, then divide by three. The correct diameter of the cockring you should order is approximately one-third of the length."
In my case, I use metal cockring.
Well, in fact, I would suggest you to get 2 of them. One is 45 mm. and another 50 mm. unless you have a huge cock, so you could try 50 and 55 mm.
Yes all package is inside cockring.
I usually wear it during the day and take it off, when I go sleep. Until now, I have no problem with my cock but only horny feeling for most of the time