The default size limit of the Exchange Server 2003 databases is 16 GB.
You can increase this value once you have installed SP-2 for Exchange Server 2003
by going to one of the following two Registry keys:

FOR THE MAILBOX STORE:
  HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\<YourServerName>\Private-objectGUID

or

FOR THE PUBLIC FOLDER STORE:
  HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\<YourServerName>\Public-objectGUID

and then creating a DWORD VALUE:
"Database Size Limit in GB" (without the quotes)

Set its value in Decimal format to be the desired upper size limit of the database, and then restart
the Information Store service.

More detail on this process can be found on the "You Had Me At HELO" site
at http://msexchangeteam.com/archive/2005/09/14/410821.aspx