Thank you Kathryn! Installing and running the Regenrate Thumbanils plug-in worked.
Kathryn, I tried that but still the featured images are too small.
Strangely, when I made a new post, the featured image did not resize.
Tahoerock: the featured image (above the post) does not fill up the space for the featured image. When I inspect the div, it mentions a resize.
