I am getting you. In all, are you trying to findout free/used space in tablespace? Finding a max extent without segment name doesn't make any use, this is what I feel. If you need to findout what I thought then use this query.
Code:
SELECT Substr(df.tablespace_name,1,20) "Tablespace_Name", 
       Substr(df.file_name,1,40) "File_Name", 
       Round(df.bytes/1024/1024,2) "Size_(M)", 
       Round(e.used_bytes/1024/1024,2) "Used_(M)", 
       Round(f.free_bytes/1024/1024,2) "Free_(M)" 
FROM   DBA_DATA_FILES DF, 
       (SELECT file_id, 
               Sum(Decode(bytes,NULL,0,bytes)) used_bytes 
        FROM dba_extents 
        GROUP by file_id) E, 
       (SELECT Max(bytes) free_bytes, 
               file_id 
        FROM dba_free_space 
        GROUP BY file_id) f 
WHERE  e.file_id (+) = df.file_id 
AND    df.file_id  = f.file_id (+) 
ORDER BY df.tablespace_name, 
         df.file_name