Show custom image sizes in the wordpress editor and media dialog. See https://since1979.dev/snippet-001-adding-custom-images-sizes/

snippet-001-add-custom-sizes-to-slze-select.php

<?php
/**
 * custom_image_sizes_in_select.
 *
 * Add custom images sizes the the size selector in
 * the gutenberg editor and the media modal.
 *
 * @see https://since1979.dev/snippet-001-adding-custom-images-sizes/
 *
 * @uses array_merge https://www.php.net/manual/en/function.array-merge.php
 * @uses __() https://developer.wordpress.org/reference/functions/__/
 */
function custom_image_sizes_in_select($sizes)
{
    return array_merge($sizes, array(
        'full-page-width' => __('Full page width'),
        'half-page-width' => __('Half page width'),
        'square' => __('Square'),
    ));
}

/**
 * Hook: image_size_names_choose.
 *
 * @uses add_action() https://developer.wordpress.org/reference/functions/add_action/
 * @uses image_size_names_choose https://developer.wordpress.org/reference/hooks/image_size_names_choose/
 */
add_filter('image_size_names_choose', 'custom_image_sizes_in_select');