How to resize tablespaces and data files

To manually increase or decrease the size of a datafile using the command.

ALTER DATABASE DATAFILE 'filename2' RESIZE 100M;

As you can change the sizes of datafiles, you can add more space to your database without adding more datafiles.
Manually reducing the sizes of datafiles allows you to reclaim unused space in the database. This is useful for correcting errors in estimations of space requirements.
Useful links :http://basic-oracledatabase.blogspot.com/2008/08/how-to-drop-tablespace-in-oracle.html and http://basic-oracledatabase.blogspot.com/2008/07/oracle-database-user-and-tablespace.html