The default database size limit for Exchange 2007 Standard Edition RTM is 50 gigabytes (GB). In Exchange 2007 Standard Edition SP1 the limit is 250GB. There is no default database size limit for the Exchange 2007 Enterprise Edition. The Exchange store process checks database size limits periodically and dismounts a database when the size limit is reached. A warning is logged using Event ID 9685 which tells you that the  database has exceeded the size limit. You can modify the database size limit by adding or changing a value in the registry.

Database Size Limit is a useful way for administrators to have control over their Exchange environment. The Exchange server will react by dismounting the store if the size gets out of hand, and it is a good way to be sure that the database size never outgrows the logical partition it resides on.

Procedure

1. To use the Registry Editor to modify a database size limit
2. Start Registry Editor (regedit).
3. Locate the following registry subkey:
   1. HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<SERVER NAME>\Private-<database GUID>
   2. You can get the GUID of a database by running the following command in the Exchange Management Shell: Get-MailboxDatabase -Identity “<server name>\<storage group name>\<database name>” | Format-Table Name, GUID
4. If the Database Size Limit in GB DWORD exists for the subkey, change its value to the desired size in gigabytes.
5. If the Database Size Limit in GB DWORD does not exist for the subkey, create a new DWORD with that name, and then set its value to the desired size in gigabytes
6. Restart the Microsoft Exchange Information Store service
7. Check evenviewer for the following events:
   1. Event ID 1216 which tells you how much size is allocated and what the current physical size is

Source: http://technet.microsoft.com/en-us/library/bb232092.aspx
See this great post about Exchange database size and limits: http://blog.sembee.co.uk/archive/2009/07/21/105.aspx