When you first start sewing, stitch length is not something you really think about. You just start sewing with the default stitch length, whether that's programmed in a computerised machine, or whatever the dial is set on for a mechanical one.
Is that a bad thing? Probably not! Depending on what you are sewing, it's a good place to start and will work for most beginner projects. But as you sew more, you'll want to vary the length of your stitches, to suit your fabric and the purpose of your stitch. Here's everything you need to know about stitch length: Read more about Stitch Length: everything you need to know