Estimate All_DB Backup Size Total

use msdb
go
/* Please change device_type and backupset type combination as you need.
backupmediafamily device_type
2 = File
5 = Tape

backupset type
L = Log
I = Differential
D = Full
*/

SELECT
      (SUM(backup_size) + SUM(1536)) / 1024 / 1024 As MBs,
      (SUM(backup_size) + SUM(1536)) /1024 / 1024 / 1024 As GBs
FROM
      backupset
INNER JOIN
(
      SELECT
            database_name,
            MAX(backup_start_date) as LastFullBackupDate
      FROM
            backupset
      WHERE
            media_set_id IN (SELECT media_set_id FROM backupmediafamily WHERE device_type = 2) AND
            type = 'D'
      GROUP BY
            database_name
) AS GetLastDate
ON
      backupset.database_name = GetLastDate.database_name AND
      backupset.backup_start_date = GetLastDate.LastFullBackupDate