If your ORACLE version is pre 8i then the process will be somewhat more complicated/ time consuming. I.e. You'll have to create new tablespaces and perform either of the following.

Option a). Create new temporary table from old.
CREATE TABLE temp AS SELECT * FROM emp;
DROP TABLE emp;
CREATE TABLE EMP AS SELECT * FROM EMP
STORAGE(...);
DROP TABLE TEMP
Great GRANTS
Recreate indexes

etc, etc.

A long process.

Option b).

Export Schema (Scott for example)
Recreate Schema objects AND indexes with correct storage sizes. The time consuming process here is recreating the DDL of the objects should you not already have it.
Import Schema back in

Ta da...