How to increase the featured image size in the post excerpt

I used Kippis 1.15 theme for a blog site, and I love the flexibility of the theme. But the person I designed the site for requests to increase the size of the featured image that shows up with the excerpt. I wasn’t able to do it with any of the standard wordpress settings in the admin panel, so I am guessing it is something that is defined and fixed in the theme. If I am right, please let me know how I can change the size of this picture.