For most homes or pro studios, length should not be that big of an issue. Even so, some cable marketing companies in an attempt to reduce the number of units they need to stock are making the claim that a specific length sounds better. It’s usually longer than typical and certainly increases their bottom line. Generally shorter is better and mainly because it saves you money, but nearly all lengths used in home audio are short. So don’t sweat it, get what works with a bit extra, particularly on loudspeaker cables. Having room to mess with loudspeaker placement is huge and generally much bigger than the effect the cables will have on performance.