As you are using our shortcode method the display of the results comes from our results.php file. You would need to customise it as described (in basic form) here:
https://searchandfilter.com/documentation/search-results/using-a-shortcode/#customising-the-results
To change the image size, edit these lines:
<?php
if ( has_post_thumbnail() ) {
echo '<p>';
the_post_thumbnail("small");
echo '</p>';
}
?>
I would suggest changing small to thumb.
You can change/delete other lines or parts. For example, to remove the date, you would need to remove this line:
<p><small><?php the_date(); ?></small></p>