It's like a sweater for your finger! This ring is made from three rows of stockinette stitch (actually three closed loops, but I won't tell if you don't). The ring has an inner circumference of 59mm, which makes it about a size 9 in North America.
2016 update: this ring is now available as a "variant", in sizes 5–12. Please see the new version of the ring on this page.