Open the registry on the Exchange 2003 server go to this key:
For Private Information Store – HKLM\System\CurrentControlSet\Services\MSExchangeIS\(Server Name)\Private-{GUID}
For Public Information Store – HDLM\System\CurrentControlSet\Services\MSExchangeIS\(Server Name)\Public-{GUID}
Under the database, create the following registry information:
Value Type: REG_DWORD
Value Name: Database Size Limit in GB
Enter a Decimal Value - For the Standard Edition of Exchange, you can enter numbers between 1 and 75. For the Enterprise Edition, you can enter numbers between 1 and 8000.
Use Exchange System Manager and Dismount the Private or Public Store in order for the changes to be applied.