Once at the Mastering Options page click on the "Stereo Width," section of the Mastering Options Menu:

From here you'll be shown the main Stereo Width Menu.

Here you can drag the bar from "Normal" to "Wider," and "Wide" to change the Stereo Width of your file to the proper setting you'd like to test:

Once you're ready to hear your changes click the "Remaster," button on the bottom left of the Mastering Options menu to re-master the file with your new Stereo Width:
​
​