The sidebar width can be fixed or percentage. This tutorial shows you how to change it in the theme Customizer. To change the sidebar width Log into your WordPress site administration panel. Navigate to Appearance > Customize, choose Sidebars. In the Sidebars panel, choose the sidebar you want to change, in this example, choose General. Under Sidebar Width, enter a valid CSS unit, e.g. 300px for a fixed width, or 30% for a percentage width.