While reviewing some of the new changes made to Exchange 2010 I came across an interesting article regarding Database Availability Groups (DAGs) and the recommended database size limits.  Microsoft has done away with Storage Groups in Exchange 2010 and have gone directly to simply databases, which gives us the highly available architecture of DAGs.  A database availability group (DAG) is the base component of the high availability and site resilience framework that is built into Exchange 2010. A DAG is a group of up to 16 Mailbox servers that host a set of databases and provide automatic database-level recovery from failures that affect individual servers or databases.   System administrators can leverage multiple mailbox servers and replicate a mailbox database to a maximum of 16 mailbox servers, as 16 servers is the maximum number of servers in a DAG.  In the event of an issue on one of the mailbox servers an Administrator can move a user’s mailbox to another database in the DAG.  This process is seamless and requires about 30 seconds.  During the transferring of the user’s mailbox to another server in the DAG the users experience no outage! 

Now, enough with the explanation, in Exchange 2010 Microsoft has suggested a maximum database size of 2TB!  The explanation:  With the significant core improvements made to Exchange 2010, the maximum recommended mailbox database size has increased from 200 GB in Exchange 2007 to 2 TB in Exchange 2010. Supporting these much larger databases means moving away from legacy recovery mechanisms, such as backup and restore, and moving to newer, faster forms of protection, such as data replication and server redundancy.  This is where the DAGs come in play.  As I mentioned above, a database can be replicated to 15 other servers in a DAG.  Keep in mind the max servers in a DAG is 16. 

I was able to find out this information and more at: http://technet.microsoft.com/en-us/library/dd638137(EXCHG.140).aspx