On the computer running Exchange Server, start Regedit.exe.
Open one of the following registry keys:

Private Database
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server name\Private-Mailbox Store GUID
Public Database
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\Server name\Public-Public Store GUID

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.