Arrow length is determined to me by two factors wether I'm shooting longbow or compound.

1: Personally, I won't cut an arrow shorter then about 1/4" past my knuckles on by bow hand. I use fixed broadheads, and this adds a bit of safety.

2: If my bow is not tuned, I start with an arrow which is longer then what I need. This can be helpful among other things in getting the correct tune from your bow. I.E. you can stiffen the reactionof your arrow by incrementally decreasing the length.