Arrow length ALONE has nothing at all to do w/ accuracy. Arrow length as it pertains to SPINE could. See, a shorter arrow will be stiffer than a longer arrow. So, let's just say you have a 27" draw length, you could shoot probably a 26" arrow. Let's just say for this discussion your spine at that length would indicate the use of a 500. But, if you have the same draw weight bow but you have a 30" draw length, obviously you are going to need a longer arrow, but not only that you'll need to use a 400, not a 500 because the arrow is 3" longer, making it not as stiff.

To sum all of that up, if you are slightly underspined you could make your arrow a tad shorter to stiffen it up a bit, and yes, THAT could affect accuracy. There is no "magical" length that will make you a better shooter. It's all totally dependant on your draw length.