SQL> select sum(bytes)/1024/1024 as Mb from dba_data_files;

        MB
----------
      2691

SQL> Col type_ heading "File type"
SQL> Col kb format 99G999G999G990 heading "Size (Kb)"
SQL> Break on report
SQL> Compute sum label "Total" of kb on report
SQL> Prompt

SQL> Select 'Datafiles' type_, round(sum(bytes/1024)) kb from dba_data_files
  2  union all
  3  Select 'Tempfiles' type_, round(sum(bytes/1024)) kb from dba_temp_files
  4  union all
  5  Select 'Redologs' type_, round(sum(bytes*members)/1024) kb from v$log
  6  /
File type       Size (Kb)
--------- ---------------
Datafiles       1,986,560
Tempfiles         276,480
Redologs           81,920
          ---------------
Total           2,344,960