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.
sum(reserved_page_count) * 8.0 / 1024 [SizeInMB]
sys.objects.name, sum(reserved_page_count) * 8.0 / 1024 [SizeInMB]
sys.dm_db_partition_stats.object_id = sys.objects.object_id
group by sys.objects.name
order by sum(reserved_page_count) DESC