Tamanho de tabela e índice no SQL Server

sp_spaceused fornece o tamanho de todos os índices combinados.

Se você quiser o tamanho de cada índice para uma tabela, use uma destas duas consultas:

SELECT
    i.name                  AS IndexName,
    SUM(s.used_page_count) * 8   AS IndexSizeKB
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.TableName')
GROUP BY i.name
ORDER BY i.name

SELECT
    i.name              AS IndexName,
    SUM(page_count * 8) AS IndexSizeKB
FROM sys.dm_db_index_physical_stats(
    db_id(), object_id('dbo.TableName'), NULL, NULL, 'DETAILED') AS s
JOIN sys.indexes AS i
ON s.[object_id] = i.[object_id] AND s.index_id = i.index_id
GROUP BY i.name
ORDER BY i.name

Os resultados são geralmente ligeiramente diferentes, mas dentro de 1%.