How short should a power cable be for my amp?

Obviously long enough to reach from outlet to your gear. I do recommend going somewhat longer than that to give you flexibility for the future

How long and how thick is cool?

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.

I have done what you are doing and in my system I found 1M to be a length that usually meets all of the requirements listed above.

Long enough to plug in. It's that simple.

There was another forum topic posted on Audiogon where people were discussing the optimal length that a power cord should be to produce the best sound. Once again I'll stick with the first comment I made on this post. BTY; I have spoken with a few major power cord manufactures and they told me power cord lengths on the short side will not impair the sound.

it must reach recepticle with no stress to the wire from where it stands.

Thanks for your responses.It's that I'm trying to clean up the wires & the power cords of my amps, are to long.

It does not make any difference on the length you decide on. Do you worry about the length of the cable from your breaker to the outlet? It's just a lengthening of that. If someone claims to hear a difference they are imagining it!

Short enough that it will span the distance between the amp and the outlet.

Why make your own cables?

In my opinion the power cord for your amp can be as short as you want to make them and I don't believe it will impair the sound.
I have no doubt others will disagree however I have compared .5/M,1/M, 1.5/M and 2/M and I could not detect a difference.