Different lenghts of CNR b'tween left/right speak

theoretically at the transmission speed of the signal, the different lengths would be negligible. however, unless you are 100% sure that you will never want to upgrade your speaker cables, i'd go with a 16 ft pair as it will be much, much easier to resell than a single 16 ft piece and a single 8 ft. piece.

Yes there is a difference with length. Always use the same length on each channel.

With the best stuff, and being able the hear the difference between cables, the length of a cable could have an effect on the sound, as diferent cables do alter the sound, so, perhaps, different lengths of the same cable can have a different effect on the sound tone.
This may or may not be true for you, and the cable you are choosing, but since reselling diferent length cables is very hard.. I would go with the same length too.
Remember, you also may decide to change stuff around, and too short is a killer... where too long is only a small annoyance.
Folks are always selling 1/2 meter cables.. and 1 meter cables.. seldom 1.5 meters or 2 meters... as those are usually always long enough. The 1/2 meter, which at the moment seems enough... always winds up too short sooner or later...

Cardas Neutral Ref is one of a VERY few cables that using different lemgths will work out well, sonically. I have several customers that have done this successfully, both with high quality s/s systems and with very revealing SET (low power) systems. I would keep the ratio of 2:1 in mind for the long/short lengths and don't go shorter than that for the best sound. The above advice for the possibility of selling the cables down the road is good advice, particularly as Cardas cables are very difficult to re-terminate and almost HAVE to be done at the factory. If you are going to keep them, however, this WILL work for you..