I use the following script:
db2 "select substr(idx.indschema,1,18) as indschema,
substr(idx.indname,1,18) as indname,idx.fullkeycard
as fullkeycard, idx.nleaf as nleaf,tbs.pagesize as
pagesize,(cast(idx.nleaf as bigint) * tbs.pagesize)/1024
as kbytes from syscat.indexes idx,syscat.tablespaces tbs where
idx.tbspaceid = tbs.tbspaceid and
idx.indschema = '<idxschema>' and idx.indname = '<idxname>'"
or
and idx.indname in ('ind1','ind2',...)