With a few exceptions, you need to use an ALTER DATABASE RESIZE DATAFILE command to resize a datafile.
Another exception is that you can use the ALTER TABLESPACE ADD DATAFILE AUTOEXTEND ON syntax to allow a database to resize itself, by growing larger as needed, with this syntax:
alter tablespace
fred
add datafile
/u01/oracle/oradata/booktst_users_02.dbf'
size 150M
autoextend on;
However to resize a datafile directly, we must use the ALTER DATABASE DATAFILE RESIZE command once again, as seen in this example:
alter database
datafile
/u01/oracle/oradata/booktst_users_02.dbf'
resize 150M;