Here is an SQL that you can run to find the total used size of an SQL Server 2008 database instance:

   CREATE TABLE #t (name SYSNAME, rows CHAR(11), reserved VARCHAR(18),    data VARCHAR(18), index_size VARCHAR(18), unused VARCHAR(18))   exec sp_MSforeachdb '      use [+];       if (select DB_NAME())<> ''tempdb''          begin             EXEC sp_msforeachtable ''INSERT INTO #t EXEC sp_spaceused [?]''          end       ', '+'    SELECT SUM(CONVERT(INT, SUBSTRING(data, 1, LEN(data)-3))) as sizeKB FROM #t     DROP TABLE #t