how to find the database size?

Database Size=sum of datafiles+sum of temporary files+sum of redologfiles+sum of controlfiles

Datafiles size:-

select sum(bytes)/1024/1024 data_size from dba_data_files;

Temporary files:-

select nvl(sum(bytes),0)/1024/1024 temp_size from dba_temp_files;

Redolog files:-

select sum(bytes)/1024/1024 redo_size from sys.v_$log;

Control Files:-

select sum(BLOCK_SIZE*FILE_SIZE_BLKS)/1024/1024 controlfile_size from v$controlfile;