So, without blabbing more, we have this error on dbfsize command:

$ dbfsize system01.dbf
system01.dbf: Header block magic number is bad; trying raw file format...
system01.dbf: Header block magic number is bad

To double check it and be sure the file is ok, you can verify it with dbv:

$ dbv FILE=system01.dbf

DBVERIFY: Release 10.2.0.4.0 - Production on Tue Feb 26 16:37:02 2013

Copyright (c) 1982, 2007, Oracle.  All rights reserved.

DBVERIFY - Verification starting : FILE = system01.dbf


DBVERIFY - Verification complete

Total Pages Examined         : 140800
Total Pages Processed (Data) : 79455
Total Pages Failing   (Data) : 0
Total Pages Processed (Index): 35549
Total Pages Failing   (Index): 0
Total Pages Processed (Other): 380
Total Pages Processed (Seg)  : 0
Total Pages Failing   (Seg)  : 0
Total Pages Empty            : 25416
Total Pages Marked Corrupt   : 0
Total Pages Influx           : 0
Highest block SCN            : 1198910981 (1.1198910981)

Solution:

The correction is simple and can be done with the RESIZE command within Oracle:

ALTER DATABASE DATAFILE '/oracle/DBPROD/system/system01.dbf' RESIZE 1100M;

Now you should have the correct output:

$ dbfsize system01.dbf
Database file: system01.dbf
Database file type: file system
Database file size: 140800 8192 byte blocks