Power cord length?

Get the exact length you need that will give the shortest distance to your outlet that won't interfere with your signal cables (and won't look like a temporary install - i.e. don't make it too short). If you have to cross over your signal cables, try do it at right angles or cut a piece of pipe insulation and put it around the power cable to maintain a gap between them. Done!

Well I like long powercords. Most of my Pangea powercords are 4 meters.
The up side is Pangea powercords are relatively cheap.
The downside on long expensive cords is they then are twice as expensive.
As the other poster mentions.. too short is a real problem.
But for an expensive cord. Consider ALL the possible locations you might want to try it and have it be long enough for that experiment.(I highly recommend trying it all over the place, never know it might really shine in an unexpected location)
Also bulky powercords can be annoying if you do not have a way to lay them out. I arranged my stuff/Power conditioner so the cords have a flat lay. If they had to be all bunched up it would be Hell.

And depending on the cord, it may have no benefit to being longer.

make sure the length of the cord is at least the distance between the outlet and the component

After that, I don't think it matters