How short should a power cable be for my amp?

If you believe that cords/cables contribute to the quality of the sound that comes from your system, then the length of a power cord can work in your favor, e.g. if you are making your cords with highly refined copper with a very continuous, smooth structure, such as Ohno Continuous Cast copper, then yes, a longer cable will provide a longer pathway of undistorted signals. Poorly drawn copper contains many boundaries in the elemental structure that interrupt, or distort, the continuous and modulated signals it carries. If you are not concerned with cables other than length and proper insulation, then making them short will give you the visual result you want. If you want to focus on the cord as a component, then it is said the last six feet are important for the refinement of your power signal.