Hmm, you can do this if you have content by using flex-grow:
.wp-show-posts-columns .wp-show-posts-inner {
flex: 1;
display: flex;
flex-direction: column;
}
.wp-show-posts-entry-summary,
.wp-show-posts-entry-content {
flex-grow: 1;
}However, you’re not using any content..
Maybe if you applied flex-grow to .wp-show-posts-image?