Webinar: Adobe Customer Journey Analytics Product Innovations: A Quarterly Overview. Come learn for the Adobe Analytics Product team who will be covering AJO reporting, Graph-based Stitching, guided analysis for CJA, and more!
Unfortunately, that isn't something we can capture automatically at this time. Neither JavaScript nor HTTP headers have access to size across multiple assets. However, if your server can track file sizes that it is sending to the browser, you may be able to set a custom variable to capture size. That would allow you to do averages.