--Below script is to get the required space for index creation, before actually it is being created.
--- Lets check for create index DBACLASS.INDEX1 on DBACLASS.EMP(EMPNO)
SET SERVEROUTPUT ON
DECLARE
v_used_bytes NUMBER(10);
v_Allocated_Bytes NUMBER(10);
BEGIN
DBMS_SPACE.CREATE_INDEX_COST
(
'create index DBACLASS.INDEX1 on DBACLASS.EMP(EMPNO)',
v_used_Bytes,
v_Allocated_Bytes
);
DBMS_OUTPUT.PUT_LINE('Used Bytes MB: ' || round(v_used_Bytes/1024/1024));
DBMS_OUTPUT.PUT_LINE('Allocated Bytes MB: ' || round(v_Allocated_Bytes/1024/1024));
END;
/