If you are doing a small tight stipple, you do want to keep the stitches smaller so the curves look like curves. If you are doing straight stitching then the stitches can be a bit larger. Remember when you are doing any curves you do need to take it slower or your stitches will "eyelash" on the back of the quilt.
FMQ on a domestic machine does take some practice. You have to get into a rhythm with your machine.
Good luck.