1. On the computer running Exchange Server, start Regedit.exe. 

2. Open one of the following registry keys: 

To configure the database size limit on a mailbox store, use the following registry key: 
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server name\Private-Mailbox Store GUID

To configure the database size limit on a public store, use the following registry key: 
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server name\Public-Public Store GUID

3. Create a new DWORD entry called "Database Size Limit in Gb" and give it the appropriate value for maximum database size in GB. 

For Exchange Server 2003 Standard Edition type a value between 1 and 75 (the default is 18 GB). 
For Exchange Server 2003 Enterprise Edition type a value between 1 and 8000 (the default is 8000 GB). Click OK. 
Note: The setting in Exchange Server 2003 Enterprise Edition is useful when you want to limit the store size, mostly due to hardware restrictions.

4. Create a new DWORD entry called "Database Size Buffer in Percentage" 
	and give it the appropriate value between 1 and 100 to specify 
	when you want events to be logged in the application log in Event 
	Viewer to warn you that you are nearing the maximum size for a database. 

	The default value of 10 (i.e. 10%) will result in warning events to be 
	logged when you have 10 percent capacity left before you reach the maximum 
	size for that database. 

5.Create a new DWORD entry called "Database Size Check Start Time in Hours From Midnight" 
	and give it the appropriate value between 0 and 23 to specify when you want Exchange to check the database size. 

The default value of 5 results in Exchange Server checking the database size 5 hours after midnight (i.e. 5:00 AM). 

6.Close Registry Editor.