You didn't mention what style of dock you have, that makes a difference: For example, a floating dock would be rigged differently than a fixed dock. A slip with a finger on both sides of the boat would also be rigged differently. You also may need to take into account the amount of tidal swing, particularly for fixed docks or bulkheads.
An interesting fact regarding line size (not length) is that the size of the dock cleat sort of determines the line diameter. Figure 1/16 inch diameter for each inch of cleat length. So for a 8 inch cleat, a 1/2 line is the best fit ( not necessarily the right capacity though) . The relationship of cleat size to line diameter is used to insure that the line tie off with the correct 'grab' If a line is sized correctly, one loop and one over and under knot is sufficient. If you have to wrap the line around the cleat several times to get it to bite, the line is too small or the cleat is too big. I many cases, the 'standard' spacing and positions of the dock cleats is often less than optimal.
For a fixed dock on one side of the boat, long crossed spring lines are the way to go.