Power cord - 5 ft vs 10 ft ?

Long enough to connect without straining, large enough to carry the required load, and as well insulated as you can afford/justify within reason.  I run 6 foot AudioQuest NRG-4 to my PS Audio Powerplants and 3 foot AudioQuest NRG-2 from the Powerplants to the components.  Sounds wonderful without breaking the bank.

It's not the length so much as the girth that's important.

Most of my power cords are of the DIY persuasion and I have a nice stash of Oyaide 046 and 004 plugs.  I bought a few hundred feet of rather good silver plated solid core copper wire some years ago and have built several power cords of varying lengths, from just a couple feet to a few meters.  I've used these on all my components over the years and yes, I have experimented to see if a short cord sounded different than a long with the same wire and plugs and I really couldn't discern a difference of any significance.  There might have been the slightest of differences but, in my systems and to my ear, it was truly negligible.

I put things comically, but I think gauge is important. Not all important, but important. There's a lot of information out there. My approach has been to try to spend enough so I was not using a cheap cord, but not so much that the cost started to be too high in proportion to my system.

I have heard demonstrations with power cables costing $2k and $7k and I could hear a difference -- in that system, that room. That didn't make me touch my wallet, however, because I want to change the weakest links, first, and I have a ways to go.

This video was interesting: 

I have heard 4 feet, but have never heard a difference in terms of length, so I just get a cord that gets me to the outlet on the power conditioner.