If you need to calculate exactly the same numbers, that are on 'table properties - storage' page in SSMS, you need to count them with the same method as it done in SSMS (works for sql server 2005 and above ... and also works correctly for tables with LOB fields - because just counting "used_pages" is not enought to show accurate index size):
;with cte as (
SELECT
t.name as TableName,
SUM (s.used_page_count) as used_pages_count,
SUM (CASE
WHEN (i.index_id < 2) THEN (in_row_data_page_count + lob_used_page_count + row_overflow_used_page_count)
ELSE lob_used_page_count + row_overflow_used_page_count
END) as pages
FROM sys.dm_db_partition_stats AS s
JOIN sys.tables AS t ON s.object_id = t.object_id
JOIN sys.indexes AS i ON i.[object_id] = t.[object_id] AND s.index_id = i.index_id
GROUP BY t.name
)
select
cte.TableName,
cast((cte.pages * 8.)/1024 as decimal(10,3)) as TableSizeInMB,
cast(((CASE WHEN cte.used_pages_count > cte.pages
THEN cte.used_pages_count - cte.pages
ELSE 0
END) * 8./1024) as decimal(10,3)) as IndexSizeInMB
from cte
order by 2 desc