The same way you change the width of a Form layout element. By using the Width setting in the Property Editor. If you set one Width, you'll want to set Width for all the Group Header, Group Body and Group Footer elements, to avoid making the browser guess at the others.

You will also see more consistent results if you set all the elements in the same column to the same width. So, if you set a Group Body element width to 200, set it's matching Group Header and Group Footer elements to the same width.