There are numerous
charts that recommend the correct size of
lines for various sizes of boats. Consult those
charts and follow their recommendations. Size does matter. Docklines should stretch, so that they absorb some of the shock when the boat rolls and draws up short on the docklines. That isn't terribly important normally, but when that 30,000# boat is bucking and rolling in it's slip during a hurricane or other severe storm, docklines that don't stretch enough can rip the
cleats out of the deck. Oversized
lines don't stretch as much as lines of correct size.