The preview window is a very useful feature of the slider. First, it shows you how the slide will look before publishing it, and second, it helps you to set the size of the slide image itself to the desired values.
In the image above you can notice that in the bottom left corner you have the size for the slider and, below it, the size for the slide. If you try to edit the height of the slide, the height of the slider will automatically adjust, so if you want to have a slide image with the height of 400px, you just specify this value in the size of the slide and you will immediately see the height that the slider itself needs to have.
Once you have the desired size for your slider, you can simply click on the 'Apply Values' button and the values of the width and height from the preview window will be copied in the 'General' panel. You can also preview the slider at the different sizes by clicking the 'Preview Size' button.