I used to execute this SQL to get size of Index in z/OS. Take a look and see it it helps you.
SELECT CREATOR,NAME,
DEC(SPACEF,11,2) AS "SPACEF IN KB",
DEC(SPACEF,11,2)/2097152 AS "SPACEF IN GB"
FROM SYSIBM.SYSINDEXES
WHERE (CREATOR IN ('XXXXXXX','XXXXXXX') OR TBCREATOR LIKE 'ABC%')
AND DEC(SPACEF,11,2) / 2097152 > 0.1
ORDER BY SPACEF DESC
WITH UR
;