Claire from Chicago ordered a ring from my humans recently. When she got it she was confused because she ordered the same size she always wears but it was too tight on her. Luckily Art and Soule offers a one-time exchange for ring size (some restrictions apply, so get with my humans for more info.) They were able to make things right for her.
Her experience is a learning experience for everyone though. You see, Claire ordered a ring that was wider than her other rings, and that's why it didn't fit the same. She had really skinny rings, and then she ordered a 10mm ring from my humans, and that's a pretty wide ring, especially for most females.
As a general rule, the wider the ring, the larger the size you need. It can be tricky to figure out just how much larger to go. You don't need to jump up several sizes or anything crazy, but if you are used to skinny rings and trying to go wider, going up a half size or even a full size (in the case of making a really big width jump) is about right. If you can, this rabbit recommends that you visit a local jewelry counter and have them size you using their WIDE sizers. Yep, those exist. Then you will have a better idea of what to order.
Oh, and if you haven't already, check out this really well-written post by the bringers of my food. They give a lot of great advice on ring sizing.
I'm gonna go chase the other rabbit now...