A quick and easy method for creating rings or bracelets to size (or any other circular shape) is to roll a piece of paper to the size you want and tape it in place. Then by securing a sturdy anchor point to part of the rolled paper you can slowly wrap your hand and the extruding plastic around the roll as many times as you like. If you move too slow it will droop and affect the shape, too fast and you will pull it apart. When you wrap it as many times as you want, secure an anchor point back on the rolled paper.  From here you extrude a line of plastic from your from top to bottom to keep all your little lines together, take it off the roll, and theres your perfectly sized circle.