Unless you want to use the Magic Loop
method you need to use a 16 in cord. If you use circs with a 16 in cord you will also need the same size DPNs when you reach the point where the stitches are stretched to far on the cord. Don't worry, using DPNs on a hat like that with established stitches is very easy.
IF you choose to do Magic Loop you won't need DPNs. You need to have at least a 32 in cord, but some prefer a 40 for ML. Some people prefer Magic Loop and that is how I learned, but I prefer a 16 in and DPNs now.