Allow your customizer to use the full width of your theme

1. In your Shopify admin click  Themes, and then click customize next to the theme on which your existing customizer is currently installed, 
I am using the dawn theme for the demonstration:

2. Go to the existing template page that your customizer currently lives on.

- On the top bar, click on the dropdown that says Home page

- In the expanded list, either in products or pages, find the template that your customizer lives on and click on that template.

3. In the Shopify template editor left-hand sidebar, click on Apps

4. Ensure that an option similar to this is selected and then save the changes for your theme.