#19 looks like the buttonhole stitch. I think #20 has 3 stitches between each "zag". As far as size, take a scrap if fabric and "play" with the settings. I usually find that the default setting is a bit too big. When you find the setting that pleases you, write the settings down for the next time.
When I got my new machine, I sat & played with most of the stitches. I used regular, light colored, rather plain quilting fabrics & tried to simulate actual conditions that I would be using the stitch under. When I weas satisfied with the stitch settings. I wrote them right on the fabric with an Ultra Fine Point Sharpie. I keep that in my sewing chest for reference.