Oracle database size is the Sum of  size of the all data files and temp files.
SELECT 'TOTAL:   '|| round(SUM(ALLOCATED_gB),2) ALLOCATED,
       'USED:   '||round(SUM(USED_gB),2) USED,
       'FREE:  '||round(SUM(FREE_SPACE_gB),2) FREE
  FROM (SELECT SUBSTR (df.NAME, 1, 40) file_name, df.bytes / 1024 / 1024 /1024 allocated_gb,
               ((df.bytes / 1024 / 1024 / 1024) - NVL (SUM (dfs.bytes) / 1024 / 1024 /1024, 0)) used_gb,
               NVL (SUM (dfs.bytes) / 1024 / 1024 /1024, 0) free_space_gb
          FROM v$datafile df, dba_free_space dfs
         WHERE df.file# = dfs.file_id(+)
      GROUP BY dfs.file_id, df.NAME, df.file#, df.bytes
      ORDER BY file_name);

ALLOCATED
-------------------------------------------------
USED
------------------------------------------------
FREE
-----------------------------------------------
TOTAL:   2256.74
USED:   1614.04
FREE:  642.7
SELECT 'TEMPSIZE:    ' ||round(SUM(USER_BYTES/1024/1024/1024),2)  TEMPSIZE FROM DBA_TEMP_FILES;

'TEMPSIZE
-------------------------------------------------
TEMPSIZE:     109.61