By default blog and posts page is set to be 3/4 for the body section and 1/4 for the sidebar section, but this may not be ideal for your layout. If you want to change the default to a wider sidebar on your blog and posts page you can use the Divi Theme customizer settings to set it to the width you want.
Steps to change the Divi sidebar width
Set the sidebar width using the Theme customizer:
- In the WordPress dashboard go to Divi > Theme Customizer > General Settings > Layout Settings
- Check the “Use Custom Sidebar Width” box and the sidebar width option will appear
- Set the “Sidebar Width” using the slider or input box (the number is a percentage of the content width which is 1080 px by default)
- Publish the changes and your sidebar width will be as you set it