Originally posted by k.chithra
Generally It means the sum of all datafiles including your system datafiles.

select sum(bytes)/1024/1024 filesize_in_MB from dba_data_files;
What abt the tempfiles in the case of locally managed temporary tablespaces???? Right query should be..

select a.bytes+b.bytes from
(select sum(bytes)/1024/1024 bytes from dba_data_files) a,
(select sum(bytes)/1024/1024 bytes from dba_temp_files) b
/