If you need to increase the width of the drop-down menus in the main navigation menu in the Divi header, the following custom CSS will allow you to do so. You can add the custom CSS code in Divi Theme options.

About the Divi WordPress Theme
Elevate your WordPress experience with the Divi WordPress Theme by Elegant Themes, a masterpiece of design and functionality. This extraordinary theme seamlessly melds user-friendly interface with stunning layouts, providing the tools for crafting captivating websites without the need for intricate coding. With its intuitive drag-and-drop builder and an expansive library of pre-designed modules, Divi empowers users to create tailored websites that adapt flawlessly to any device, making it a cornerstone of modern web development.