Calculate all Datafile Size Individually

Whenever you say the size of the database it means the summation of the datafiles. An oracle database consists of data files, redo logfiles, control files, temporary files. Database's size comes actually from these datafiles. To know about the size of all datafiles:

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

Size of all Temp Files:
SQL>select sum(bytes)/1024/1024 "Meg" from dba_temp_files;

Size of the on-line redo-logs:
SQL>select sum(bytes)/1024/1024 "Meg" from v$log;

Size of the control files:
SQL>select sum(BLOCK_SIZE*FILE_SIZE_BLOCK/1024/1024) "Meg" from v$controlfile;

To get the total size of the database:
select a.data_size+b.temp_size+c.redo_size+d.controlfile_size "total_size in MB"
from ( select sum(bytes)/1024/1024 data_size
from dba_data_files ) a,
( select nvl(sum(bytes),0)/1024/1024 temp_size
from dba_temp_files ) b,
( select sum(bytes)/1024/1024 redo_size
from sys.v_$log ) c,
( select sum(BLOCK_SIZE*FILE_SIZE_BLKS)/1024/1024 controlfile_size
from v$controlfile) d;