If you are referring to Free Motion Quilting (FMQ), you drop your feed dogs and let the movement of the fabric and speed of the machine dictate the stitch length. Once you drop the feed dogs, the stitch length is irrelevant. The feed dog motion sets the length. When quilting straight lines using a walking foot, set it to your normal length or slightly longer (2.5mm-3.5mm). The length you choose depends on your own taste. Try a few different lengths on a practice piece and go with what looks nice to you.