To get accurate size of the database, We have to combined the sizes of all datafiles, tempfiles and redologfiles.

select round ((a.data_size+b.temp_size+c.redo_size)/1024/1024/1024,2) “DB_size_in_GB”
from ( select sum(bytes) data_size
from dba_data_files ) a,
( select nvl(sum(bytes),0) temp_size
from dba_temp_files ) b,
( select sum(bytes) redo_size
from sys.v_$log ) c;