i don't even bother the stitch length anymore. your speed of moving the quilt determines the stitch length. i took a class after doing FMQ for some time and the instructor said not to lower the feed dogs. we didn't. but i've put them down mostly since. depends on the fabric, batting, etc as to whether you encounter a problem or not.
just drop the feed dogs , foot/needle lever down and put on a darning foot with a spring on the shaft and quilt away. steady fast foot pedal speed and steady slower movement of your quilt.