This Code Hack will help you adjust the featured image size in the blog post feed. You must have retina images enabled in the Welcome tab, and have regenerated thumbnails for this Code Hack to work. You will be adjusting the width of the featured image, and it will scale proportionally. Note that you will also need to adjust the width of the content to maintain spacing between the two.
Here’s a demo showing you what this Code Hack does:
And here’s the code you need to make it happen: