One can manually increase or decrease the size of a datafile from
Oracle 7.2 using the command.
ALTER DATABASE DATAFILE 'filename2' RESIZE 100M;
Because you can change the sizes of datafiles, you can add more
space to your database without adding more datafiles. This is
beneficial if you are concerned about reaching the maximum number
of datafiles allowed in your database.
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.
Also, datafiles can be allowed to automatically extend if more space is
required. Look at the following command:
CREATE TABLESPACE pcs_data_ts
DATAFILE 'c:\ora_apps\pcs\pcsdata1.dbf' SIZE 3M
AUTOEXTEND ON NEXT 1M MAXSIZE UNLIMITED
DEFAULT STORAGE (INITIAL 10240
NEXT 10240
MINEXTENTS 1
MAXEXTENTS UNLIMITED
PCTINCREASE 0)
ONLINE
PERMANENT;