Link to original Microsoft article:
http://support.microsoft.com/kb/912375/

How to increase the Exchange Server 2003 Service Pack 2 database size limit

To increase the Exchange Server 2003 SP2 database size, follow these steps.

Important Before you increase the maximum size of an Exchange database, verify that sufficient hard disk space is available for the larger database.

1. On the computer that is running Exchange 2003 SP2, click Start, click Run, type regedit, and then click OK.
2. Click one of the following registry subkeys, as appropriate for the store that you want to increase:
   • For a mailbox store, click the following registry subkey:
     HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server name\Private-Mailbox Store GUID
   • For a public folder store, click the following registry subkey:
     HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server name\Public-Public Store GUID
3. On the Edit menu, point to New, and then click DWORD Value.
4. In the New Value #1 box, type Database Size Limit in Gb, and then press ENTER.
5. Right-click Database Size Limit in Gb, and then click Modify.
6. Click Decimal, and then type an integer from 1 to 75 in the Value data box.
   
   Note These integer values represent the maximum size of the database in gigabytes (GB). For example, a value of 75 represents a database that has a maximum size of 75 GB.
7. Click OK, and then exit Registry Editor.
8. Restart the Microsoft Exchange Information Store service. To do this, follow these steps:
   1. Click Start, click Run, type cmd, and then click OK.
   2. At the command prompt, type the following command, and then press ENTER:
      net stop msexchangeis
   3. After the Microsoft Exchange Information Store service has stopped successfully, type the following command, and then press ENTER:
      net start msexchangeis
9. Examine the Application log to verify that the database size has been set successfully. To do this, follow these steps:
   1. Click Start, click Run, type eventvwr, and then click OK.
   2. In the Event Viewer tool, click Application.
   3. Double-click event ID 1216 to verify that the database size has been set successfully.

To determine if you're running Exchange Server 2003 SP2:

1. Open Exchange System Manager
2. Expand 'Servers'
3. Right-click on [server name], choose Properties
4. On the General tab, verify that the version is Version 6.5 (Build 7638.2: Service Pack 2)