Definir tamanho dos thumbs da [gallery] WordPress

gistfile1.txt

function amethyst_gallery_atts( $out, $pairs, $atts ) {
   
    $atts = shortcode_atts( array(
        'columns' => '2',
        'size' => 'medium',
         ), $atts );
 
    $out['columns'] = $atts['columns'];
    $out['size'] = $atts['size'];
 
    return $out;
 
}
add_filter( 'shortcode_atts_gallery', 'amethyst_gallery_atts', 10, 3 );