It depends on what arrow you're using not the rest (although you don't want to be shorter than the rest for obvious reasons). Arrow length determines arrow spine. The shorter the arrow the stiffer the spine, the longer the arrow the weaker the spine. Arrow spine is one of the most influential items for arrow groups. Look at the arrow charts for the Mfg. of your arrow. They are a good starting place to determine length. You might have to adjust from there. It's ok to have a long arrow if it is the correct spine.