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.