wp-selectable-image-size.php

<?php
/**
 * Make custom images sizes selectable
 *
 * @param array $sizes
 * @return array
 */
function op_selectable_image_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'twentyseventeen-thumbnail-avatar' => __( 'Thumbnail Avatar', 'twentyseventeen' ),
    ) );
}
add_filter( 'image_size_names_choose', 'op_selectable_image_sizes' );