You can try adding this PHP snippet.
add_filter( 'generate_featured_image_output', function(){
return sprintf(
'<div class="post-image">
%3$s
<a href="%1$s"><img class="author-image" src="%2$s"></a>
</div>',
esc_url( get_permalink() ),
lh_page_hero_gravatar(),
apply_filters( 'generate_inside_featured_image_output', '' )
);
});
function lh_page_hero_gravatar() {
global $post;
$author_id = $post->post_author;
return get_avatar_url( $author_id,['size' => '400'] );
}
Here’s how to add PHP – https://docs.generatepress.com/article/adding-php/
You can adjust the size by changing the 400 to a value of your preference.