
SELECT meta_key, 
COUNT(*) AS `Rows`,  
  ROUND(
    SUM(
      LENGTH(
        CONCAT(meta_id, post_id, meta_key, meta_value)
      )
    )/1048567, 2
  )
AS `Size_in_MB`
-- Refer $table_prefix
FROM wp_postmeta
GROUP BY meta_key
ORDER BY `Size_in_MB` DESC;