functions.php

<?php

/**
 * Bulk update posts 
 *
 */
function be_bulk_update_posts() {
  $loop = new WP_Query( array(
    'posts_per_page' => 100,
    'meta_query' => array(
      array(
        'key' => 'be_featured_image_width',
        'compare' => 'NOT EXISTS',
    ) )
  ) );
  
  if( $loop->have_posts() ): while( $loop->have_posts() ): $loop->the_post();
    if( has_post_thumbnail() ) {
      $image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'full' );
      if( !empty( $image[1] ) )
        update_post_meta( get_the_ID(), 'be_featured_image_width', intval( $image[1] ) );
    }
  endwhile; endif; wp_reset_postdata();
}
add_action( 'wp_footer', 'be_bulk_update_posts' );