Estimating the Size of an Index

SET SERVEROUTPUT ON
declare
l_index_ddl VARCHAR2(1000);
l_used_bytes NUMBER;
l_allocated_bytes NUMBER;
BEGIN
DBMS_SPACE.create_index_cost (
ddl => 'create index FIN_EDATT_DEPENDENTS_AS_DEP_ID on FIN_EDATT_DEPENDENTS_AS_DEP(STUDENT_ID_NUM)',
used_bytes => l_used_bytes,
alloc_bytes => l_allocated_bytes);
DBMS_OUTPUT.PUT_LINE ('used = ' || l_used_bytes || 'bytes'
|| ' allocated = ' || l_allocated_bytes || 'bytes');
 END;