Inner diameter is 21 mm (0.83 inch). You can scale it to fit your finger.

Print the object with the pentagram facing down toward the printing plate. Different colors are created by color changes during printing. Look how to set color change in your slicer. In the prusa slicer, slice the object, then move the vertical slider on the right side of the preview (which is orange in my case) down to the layer of color change and press the + next to it. Repeat this for several colors. Then press "slice now" again.

To make the ring look more like real metal you can use Proto Pasta Iron PLA, which you can let rust. Since this filament is not so stable, I recommend in this case an infill of 30 - 50 % and a wall thickness of at least 4 at 0.1 mm resolution.

For normal PLA, 10% infill and 3 layers of wall thickness at 0.1 mm resolution is sufficient.