An excellent way to figure out exactly what size you need for both effectiveness and comfort is to get an assortment of sizes in neoprene or Buna-N industrial O-rings and experiment a little. They cost much less than a dollar apiece so a full assortment won't break the bank, and they come in diameter increments of 1/16 inch. Metric sizes are also available, making even more size variability possible. You can also get O-rings in Teflon as seen in my first attachment here. I found that the slipperiness of the smooth Teflon makes them more comfortable than rubber in a tighter fit. I also have a stainless steel ring, second attachment, which is similarly more comfortable because it doesn't pull and drag on the skin like rubber. I've just recently picked up a neoprene ring, 3rd attachment, which is very stretchy and more comfortable than most rubber rings. You can check that one out at:
Mr. S Leather -- Neoprene Cock Ring
A 2" i.d. is a little too tight for me, once erect my cock turns blue and I wonder if I'll ever get the damned thing off, but this neoprene ring is so stretchy that it's easy to stretch it out to a larger size when putting it on and taking it off. I do find that if I'm going to wear it for any length of time, and especially if I'm going to engage in vigorous sex, it helps to lube the edge where it meets the back of my cock shaft and scrotum. They also have a pure silicone ring that is even more stretchy:
Mr. S Leather -- Silicone Cock Rings
But also notably more expensive. For the most part I've been using the SS ring which has a classier look. I've found that a 2-1/16 inch i.d. is just about optimum with the Teflon rings but I bought the SS ring in a 2-1/8 i.d. because, unlike the O-rings, there's no option of snipping it off if ever I had serious difficulty getting out of it.
Good luck.
