How to Find Over-all Database Size?
Big Portion of the database size comes from Datafiles.
To find out the size allocated to datafiles you can use below mentioned query :
1) select sum(bytes)/1024/1024 "Meg" from dba_data_files;
To get the size of all Temporary Files :
2) select nvl(sum(bytes),0)/1024/1024 "Meg" from dba_temp_files;
To get the size of Online Redo Log Files :
3) select sum(bytes)/1024/1024 "Meg" from sys.v_$log;
So counting all of them you will get the size of the Database.