1.
sqlplus SYS AS SYSDBA
2.
SELECT file_name, tablespace_name, bytes/1024/1024 UNDO_SIZE_MB, SUM(bytes/1024/1024) OVER() TOTAL_UNDO_SIZE_MB FROM dba_data_files d WHERE EXISTS (SELECT 1 FROM v$parameter p WHERE LOWER (p.name)='undo_tablespace' AND p.value=d.tablespace_name);
3.
alter database datafile '/oracle/ora10g/ecms/syscom01.dbf' resize 4G;
Note: It is not recommended to grow any data files above 20 GB in size. If a tablespace needs to be grown over 20 GB a new datafile has to be added.
source : https://www.ibm.com/docs/en/tnpm/1.4.4?topic=administration-resize-undo-tablespace
https://forums.oracle.com/ords/apexds/post/resize-datafile-5440
