The size of the database is the total size of the datafiles that make up the tablespaces of the database. These details are found in the dba_extents view. type the following lines at the SQL*PLUS prompt:
######## datafiles size ##########
SQL> select sum(bytes/1024/1024/1024) from dba_data_files;
######## actual database size #########
SQL> select sum(bytes/1024/1024/1024) from dba_segments;
The size returned is in GB