In the Debutify Shopify theme, you can change the width of the Newsletter pop-up to convert visitors to subscribers and buyers. When deciding on the width, bear in mind how this will look on a desktop screen compared to a mobile phone.
How to Change Width of Newsletter Pop-up
Go to your Shopify Admin, and select Online Store.
Select your Debutify Shopify theme, and click on Customize.
Click on Theme Settings.
Click on Newsletter Pop-Up.
Under Pop-Up settings, select the width of the newsletter pop-up.