Extending Vajk Hermecz’s answer.
This is how you can get all of the indexes size, in megabytes, without the PRIMARY (which is the table itself), ordered by size.
SELECT database_name, table_name, index_name,
ROUND(stat_value * @@innodb_page_size / 1024 / 1024, 2) size_in_mb
FROM mysql.innodb_index_stats
WHERE stat_name="size" AND index_name != 'PRIMARY'
ORDER BY size_in_mb DESC;