Webpage size refers to the total file size of all assets that load when a web page is accessed, including HTML, CSS, JavaScript, images, videos, and other multimedia. A larger webpage size can slow down load times, negatively impacting user experience and SEO rankings.

Optimizing webpage size involves compressing images, minimizing scripts, reducing unused CSS, and implementing lazy loading to ensure assets load efficiently. Keeping webpage size in check is crucial for improving web performance and meeting user expectations for quick access to content.