BREAK ON REPORT
COMPUTE SUM LABEL TOTAL OF bytes ON REPORT
select file_name, bytes from dba_data_files;

FILE_NAME                                                         BYTES
------------------------------------------------------------ ----------
D:\ORADATA\DBNAME\SYSTEM01.DBF                                492830720
D:\ORADATA\DBNAME\UNDOTBS01.DBF                              1080033280
D:\ORADATA\DBNAME\SYSAUX01.DBF                                251658240
...
D:\ORADATA\DBNAME\TOOLS01.DBF                                 104857600
D:\ORADATA\DBNAME\TOOLSAUDIT.DBF                              288358400
D:\ORADATA\DBNAME\USERS01.DBF                                 555745280
                                                             ----------
TOTAL                                                        1.3980E+11

25 rows selected.