I am really so impressed with the overall quality of the King Rings. I initially purchased mine and used the sizing rings as a guide. I highly recommend using the sizers as these guys are not cheap items you want to toss in a drawer and forget. Where the sizing rings require a bit consideration is they do not replicate the weight of the actual product.
As a really new customer, I want to offer just a bit from what I've learned. For starts I am in Arizona where it is hot and I usually swim multiple times a day. In my case I wasn't that In-tune of how my anatomy adjust to the temperatures. I ordered two Cock Rings in different sizes. The larger of the 2 (2+ fingers) is my go-to as it is very comfortable and other than the pleasure of the weight, I don't notice it. The downside is that if I swim or spend time in the heat it begins to slide down and in some cases I remove it so my secret wouldn't drop to the floor and roll around.
The smaller of the two stays put, in these situations, does better and as comfortable but any minor arousal becomes a bit more complete but isn't necessarily prolonged.
I have some rings from another manufacturer and they come in at 55mm (their largest size) and are at times a bit uncomfortable due to 90 degree transitions and much narrower width.
Today I ordered a 54mm True Blue. This will be my around the house ring when my Ladie and I are hanging out :-)
This is a long winded description really meant to encourage some thought. In my situation I will have 3 (maybe more) King Rings of different sizes intentionally. If in doubt, read the FAQs and BLOG and if this doesn't answer your question, Scott is superior at customer service.
-rp