Thank you Sreddy for your last post. I wanted to investigate the matter myself. This is what I found in 'Understanding the Oracle Server' book by M.Krakovsky:
'To minimaze data fragmentation, the Oracle7 Server allocates space for data files when you create a database. If the Server did not preallocate space, and simply took disk space on an as-needed basis, it could not guarantee that related data-such as data used by a single application-would be stored contiguously.
In order for the Server to allocate space for a data file, you must tell it how much to allocate, using the CREATE DATABASE command. For example, to created a PUBREL database whith an initial data file of 20mb, you would enter a command containin the following lines:
SQLDBA> CREATE DATABASE pubrel
..
..
DATAFILE 'usr/pr/datafile1.dbf' SIZE 20M;

Correctly setting the size of each data file using the CREATE DATABASE command is important because you may not be able to change it later.'.....