SELECT OWNER,SEGMENT_NAME,BYTES/1024/1024 AS SIZE_MB FROM DBA_SEGMENTS
WHERE 1=1
AND OWNER = 'USER1'
ORDER BY 1;

SELECT --FILE_NAME, BYTES, 
* FROM DBA_DATA_FILES 
--WHERE TABLESPACE_NAME = 'USERS'
;

ALTER DATABASE DATAFILE 'D:\ORACLEXE\APP\ORACLE\ORADATA\XE\UNDOTBS1.DBF'
AUTOEXTEND ON NEXT 8 MAXSIZE UNLIMITED; --512K