Post Thumbnail Image Size

Currently it uses get_the_post_thumbnail() with the default post-thumbnail size param:
https://developer.www.ads-software.com/reference/functions/get_the_post_thumbnail/

So one option would be adjusting the thumbnail size in WordPress Dashboard > Settings > Media > Thumbnail size. That would be site-wide, though.

Another option would be using a custom template and specifying the size you want, something like:

<?php
/*
Template Name: Insert Featured Image
*/

while ( have_posts() ) {
    the_post(); 
    ?>
    <a href="<?php echo esc_url( get_permalink() ); ?>"><?php echo get_the_post_thumbnail( null, array( 1024, 768 ) ); ?></a>
    <?php 
}