Any size will root. I personally prefer cuttings that are smaller in diameter than my thumb and about 7"-8" in length with at least two full nodes.
As a rule of thumb, I always cut a quarter inch BELOW the joint on the bottom of the cutting and a quarter inch ABOVE the joint on the top of the cutting. Think of the space between joints as an "energy container", and each joint (line) on the cutting is like the end cap of the container. If you breach the container (the joint), the pith inside will eventually rot and the cutting will hollow out.