Best way to add image sizes

In case someone needs that as well. I did it like that. Add this to the child themes functions.php

/* additional image size(s)
 *
 */

// add new size
add_image_size( 'bigger-square', 250, 250, true );

// make the new size available in the media library (and image element etc.)
add_filter( 'image_size_names_choose', 'my_custom_sizes' );

function my_custom_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'bigger-square' => __( 'Bigger Square' ),
    ) );
}