The following CSS will work. I encourage you to create your own Child Theme (https://codex.www.ads-software.com/Child_Themes) based on Hoffman. It will only take a few extra steps, but can save you a lot of time moving forward, especially if you intend to make any additional changes:
.featured-media img {
width:100%;
}
A few caveats: This will enlarge your images to fit the width of the content area (1200px). If you use a very small, low-resolution image to begin with, it probably won’t look the greatest when enlarged. There may also be a specific reason the theme’s author did not choose to do this, so make sure you do some testing to ensure that everything still functions as expected.
Let me know if you need help getting the child theme set up. I’m happy to walk you through it.