In responsive websites, the site layout is designed to resize and flex to fit the browser, regardless of size. Usually, the width and height of items in your layout will be sized automatically, based on the stuff inside them, growing as wide or tall as needed.
Sometimes you want to control the width or height of items to achieve a particular look. With the appearance controls ProPhoto’s Customizer provides, you can change the default, automatic sizing of some layout items.
Width of layout items