you can try using oncheck -pt <db_name>:<db_owner>.<table_name> <cr> from the command line and you'll see a section of values for each index after the data section of the output. Though you can specify an First extent size and next extent size when creating a table, I don't know how the First extent size and next extent size values are derived for indexes.
AIX_production > oncheck -pt FIN8.4_Live:sysadm.ps_employee <cr>
In the section for each index you'll see values called "Number of Pages Allocated" and "Number of Pages Used", so use whichever value is more meaningful to you and multiply by the page size (AIX has 4k pages) and you'll have a pretty good idea how much space each index is using.