I've found the whole process goes quicker based on diameter of cut not length. I try to look for cuts that are at least 1/8 inch thick. That said I cut all mine at about 6 inches making sure a node is underground.
I've taken cuts as short as a couple of inches and as long as five to six inches. The biggest factor whether a cut roots or not comes down to health, the healthier the cut the better chance it has to root.