I used about the same sizes as with the wire ring I made first. The following part of text gives an explanation of what this is and why you would use it. The file is public, so you can modify it to make it fit for yourself. The instructable includes how to change it to your own size.
I hope you like it!
The idea behind it came when I was reading a book. Well actually, the idea came the moment I put the book down. I couldn't find my page marker, although I was sure I had it a minute ago. So I thought, how can I make sure I have my page marker, while reading the book, without risking to lose it? Of course: wear it! And, a small ring would give about the right size.
The ring has a total height of 0,22 cm. Reason it is so low is that it has to be in the book. The gap between the two parts is 0,02 cm, enough for any kind of paper. The two parts are held together with the circle on front and a small row of cylinders, also on the front. This will give just enough support for the rings to be one piece.
The 3D model can be found in the last step.
If you like this idea, I would really appreciate a vote in the 123D Autodesk contest.
Step 1: What to find where
Step 7/8 How to modify the 3D model
Step 9 Where to find the 3D model