Adjusting Stitch Width
On the straight stitches, the stitch width is 0. Stitch length can be effected on the straight stitch, but width is 0 because the needle is going in a straight line. Adjusting the width will only move the needle left or right, but you can achieve the same effect by pressing a different starting stitch.
But on a zig-zag stitch, adjusting the stitch width will effect the width of the zig-zag. Default is 3.5mm, but the width can be dropped to 0, effectively making it a straight stitch, or upped all the way to 7mm, making it almost a basting zig-zag stitch. All of these adjustments are from the sewing/stitch select screen.
Along with adjusting the stitch width, if you have a preferred width for your zig-zag, you can press the memory button and The Machine will save your preference. There is also a display button you can press which will show you what the stitch will look like. All of this is demonstrated on the video: