Oracle DB Size :- Normally the size of the database is denoted by total size of the physical data files. This query will help you to manage the space occupied by your database.

get_dbsize_oracle.sql

clear breaks 
clear computes 
clear columns 
set pagesize 50 
set linesize 120 
set heading on 
column tablespace_name heading 'Tablespace' justify left format a20 truncated 
column tbsize heading 'Size|(Mb) ' justify left format 9,999,999.99 
column tbused heading 'Used|(Mb) ' justify right format 9,999,999.99 
column tbfree heading 'Free|(Mb) ' justify right format 9,999,999.99 
column tbusedpct heading 'Used % ' justify left format a8 
column tbfreepct heading 'Free % ' justify left format a8 
break on report 
compute sum label 'Totals:' of tbsize tbused tbfree on report 
select t.tablespace_name, round(a.bytes,2) tbsize, 
nvl(round(c.bytes,2),'0') tbfree, 
nvl(round(b.bytes,2),'0') tbused, 
to_char(round(100 * (nvl(b.bytes,0)/nvl(a.bytes,1)),2)) || '%' tbusedpct, 
to_char(round(100 * (nvl(c.bytes,0)/nvl(a.bytes,1)),2)) || '%' tbfreepct 
from dba_tablespaces t, 
(select tablespace_name, round(sum(bytes)/1024/1024,2) bytes 
from dba_data_files 
group by tablespace_name 
union 
select tablespace_name, round(sum(bytes)/1024/1024,2) bytes 
from dba_temp_files 
group by tablespace_name ) a, 
(select e.tablespace_name, round(sum(e.bytes)/1024/1024,2) bytes 
from dba_segments e 
group by e.tablespace_name 
union 
select tablespace_name, sum(max_size) bytes 
from v$sort_segment 
group by tablespace_name) b, 
(select f.tablespace_name, round(sum(f.bytes)/1024/1024,2) bytes 
from dba_free_space f 
group by f.tablespace_name 
union 
select tmp.tablespace_name, (sum(bytes/1024/1024) - sum(max_size)) bytes 
from dba_temp_files tmp, v$sort_segment sort 
where tmp.tablespace_name = sort.tablespace_name 
group by tmp.tablespace_name) c 
where 
t.tablespace_name = a.tablespace_name (+) 
and t.tablespace_name = b.tablespace_name (+) 
and t.tablespace_name = c.tablespace_name (+) 
order by t.tablespace_name
;