gnagesh,
Thanks for the answer.
RESIZE is ok.

But suppose I created the original Datafile as

ALTER TABLESPACE ORGSDATA ADD DATAFILE 'F:\\ORGS\\DBFILES\DATA02ORGS.DBF' SIZE 100M

Now I am trying to add a new datafile using the options
AUTOEXTEND ON NEXT 10K MAXSIZE UNLIMITED

In such a case if I want to change the existing datafile to satisfy my needs...can I do it?Or in this case I HAVE to add a fresh datafile.
Further what is the usefulness of REUSE clause?