For most applications, a 40 ft. wrap on a 2-inch shaft is plenty. If you might use the oars on boats of differing width, a 50 ft. wrap will be more than enough. Just make sure you set the oars in the locks with the boat rigged, so you get them marked right. Most of the wrap should be outboard of the lock, with 3-4 inches inboard of the donut.
Best is to make it a two-person gig, with one handling the rope and one rotating the oar. Really reef on it—you can't get it too tight by hand.