functions.php

<?php
/**
 * Custom image sizes
 */
add_image_size('my-custom-image-size', 768, null, false);

add_filter( 'image_size_names_choose', function( $sizes ) {
  // Remove build in sizes
  unset($sizes['thumbnail']);
  unset($sizes['medium']);
  unset($sizes['large']);

  // Add the custom image size
  $sizes['my-custom-image-size'] = 'My Custom Image Size';

  return $sizes;
} );