I've read that longer is better [ well just ask the ladies :-) ] but seriously now...
Why longer lengths are considered to sound better I don't understand, but that's how those who have experimented have answered this question. I recall that it does relate to the filtering action of the longer length being more effective. How much longer & how much difference(?) is better answered yourself, in your own rig, with a particular cord connected to a particular component. Order a 6' length of Power 3 for comparison sake would be my suggestion. Any other cable, of any other length, in any other rig, is not necessarily legitimately comparable with your own results.
Why longer lengths are considered to sound better I don't understand, but that's how those who have experimented have answered this question. I recall that it does relate to the filtering action of the longer length being more effective. How much longer & how much difference(?) is better answered yourself, in your own rig, with a particular cord connected to a particular component. Order a 6' length of Power 3 for comparison sake would be my suggestion. Any other cable, of any other length, in any other rig, is not necessarily legitimately comparable with your own results.