The first one here will provide you with the size of your database in MB.
The second will also do the same, but it will break them out for each of the objects in your DB ordered by largest to smallest.
select
sum(reserved_page_count) * 8.0 / 1024 [SizeInMB]
from
sys.dm_db_partition_stats
GO
select
sys.objects.name, sum(reserved_page_count) * 8.0 / 1024 [SizeInMB]
from
sys.dm_db_partition_stats, sys.objects
where
sys.dm_db_partition_stats.object_id = sys.objects.object_id
group by sys.objects.name
order by sum(reserved_page_count) DESC