You can add the CSS to the custom CSS box in Divi > Theme Options, or add to your child theme. Once you do this, you can remove the CSS code section from the page.
All CSS is commented for easy editing. The excerpt length is limited with CSS at certain screen sizes using media queries. If you change fonts or font sizes you may need to adjust these media queries for your specific needs.
You can check this Quick Snack for more info on how this is achieved https://divisoup.com/q17-limit-the-excerpt-length-in-divi-with-css/