A size of all Exchange databases is displayed as 0 KB or blank in Backup Selections. (Figure 1, Figure 2 and Figure 3)
Figure 1 - Exchange 2007 databases and Backup Exec 2010 R3
Figure 2 - Exchange 2010 databases and Backup Exec 2010 R3
Figure 3 - Exchange 2007 and Backup Exec 2012
This is by design. There is no request to determine the size of the databases before running backups.
The size can be ignored. If the size of the databases is needed, please see in the job log after backups complete.
Exchange 2003, Exchange 2007, Exchange 2010