Unused Index Sizes (GB)

  SELECT a.owner,
         a.index_name,
         a.table_name,
         a.monitoring,
         a.used,
         a.start_monitoring,
         SUM (b.bytes) / 1024 / 1024 / 1024 GB
    FROM all_object_usage a, dba_segments b
   WHERE a.index_name = b.segment_name AND a.used = 'NO'
GROUP BY a.owner,
         a.index_name,
         a.table_name,
         a.monitoring,
         a.used,
         a.start_monitoring
ORDER BY GB DESC;