Posted by “Strange” things happen when you take a tablespace or its datafile(s) offline.
SQL> create tablespace TS datafile 'C:\ORACLE\ORADATA\NP12\TS.DBF' size 20m;
Tablespace created.
SQL> alter tablespace TS offline;
Tablespace altered.
SQL> select bytes from dba_data_files where file_name = 'C:\ORACLE\ORADATA\NP12\TS.DBF';
BYTES
----------
We lose access to the file size. So how can you tell how large that file is, without jumping into the OS or bringing the tablespace online again ?
We can treat the file as a blob.
SQL> create or replace directory DF as 'C:\ORACLE\ORADATA\NP12';
Directory created.
SQL> set serverout on
SQL> declare
2 b bfile := bfilename('DF','TS.DBF');
3 begin
4 dbms_output.put_line(dbms_lob.getlength(b));
5 end;
6 /
20979712
PL/SQL procedure successfully completed.
Easy peasy 
Learning is not a spectator sport 
Unforunaltely this does not work with ASM
sokrates@12.1 > create tablespace TS datafile '+DATA/TS.DBF' size 20m; Tablespace created. sokrates@12.1 > select bytes from dba_data_files where tablespace_name='TS'; BYTES ---------- 20971520 sokrates@12.1 > alter tablespace TS offline; Tablespace altered. sokrates@12.1 > select bytes from dba_data_files where tablespace_name='TS'; BYTES ---------- sokrates@12.1 > create or replace directory DF as '+DATA'; Directory created. sokrates@12.1 > set serverout on sokrates@12.1 > declare sokrates@12.1 > b bfile := bfilename('DF','TS.DBF'); sokrates@12.1 > begin sokrates@12.1 > dbms_output.put_line(dbms_lob.getlength(b)); sokrates@12.1 > end; sokrates@12.1 > / declare * ERROR at line 1: ORA-22288: file or LOB operation GETLENGTH failed No such file or directory ORA-06512: at "SYS.DBMS_LOB", line 850 ORA-06512: at line 4Ah….Good point. I wonder if there is a similar means possible (aka without heading into V$ territory)