SELECT
    i.name                  AS IndexName,
    SUM(s.used_page_count) * 8 /1024  AS IndexSizeMB
FROM sys.dm_db_partition_stats  AS s 
JOIN sys.indexes                AS i
ON s.[object_id] = i.[object_id] AND s.index_id = i.index_id
WHERE s.[object_id] = object_id('dbo.document_line')
GROUP BY i.name
ORDER BY i.name