Power Cord Length

20m or 2m no difference. length of powerline is much longer could be 2km 
The efficiency of current has nothing to do with length of power cord.
Longer ones are usually more convenient to reach to power outlet from the placement position. I've seen those super fat ones that you can hardly bend, but "sounding super good" LOL!  If you can't even bend them steep enough to reach the outlet, what's the use? 
The rule of thumb is super-simple, if you still unsure how aftermarket power cord works or how will one work in your set up, there's no need to bother and use stock one and save $$$. I still don't know why I should change my power cords unless they break and live with that with no regrets whatsoever.  
Many high-end components might not have sufficient "defense" from EMI of powerlines and may require to use SHIELDED powercord in case if such problem NOTED. See? I'm not completely against aftermarket powercords after all, but highly prefer well-engineered components that aren't susceptible to EMI, RFI or other electrical nature disturbances. 
A representative will always tell you to purchase because it will improve sound, but in fact not always true and most-likely not true. 

As a rule good cables don't improve the signals going through them, they just don't negatively impact as much as lesser cables.

Have never heard that the length on a power cable is critical. Their is a lot of discussion on the proper length of digital cables though.
Perhaps the Mfg was referring to some harmonic resonance frequency of the 60hz power wave?
I have heard a definite difference in the sound with some power cables and virtually no change with others.
An audition in your system is a must. Many mfg's have a trial period and the Cable Co has their Library program. 
Good luck, enjoy the journey and trust your ears.

Maybe because at the factory where they're made, the workstations are five feet apart and one worker does the input side while the other does the output side. They call that their "current" configuration: the current workstations can be no closer than five feet apart. Ergo, the current has to be five feet, minimum.

dbtom2- not bad...
Hopefully, almarg will post on this. He usually has the ability to answer these questions with scientific acumen.
In my humble opinion, I would use a power cord that is long enough to reach the outlet and component, yet provide enough movement so that adjustments to interconnects can be done with ease.
I went from PS Audio PC's to Zu. The PS were a P in the A, due to their thickness and inflexibility. The Zu's are more flexible, but only come in 1+ meter lengths-shorter lengths are priced at the 1 meter price-so I now have an extra half meter for some of my power cables.
As far as sound goes, the PS cables sounded better than a stock cable, but the Zu's didn't seem to add or detract from the overall sound. So, I guess there is a point of diminishing returns with power cables.
YMMV
Bob