Increase size of MS Outlook PST File

How do I increase the maximum file size of an UNICODE pst file? The default size is 20 GB but as far as I know there must be a registry value that allows to increase the maximum up to approximately 4TB?

Yup, you can do this via the registry or via Group Policies. Note that this does not apply to pst-files created in Outlook 97-2002 (ANSI formatted pst-files).

Via the Registry
You need to go to the following location in the registry;
Outlook 2003: HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\PST
Outlook 2007: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\PST

Here you must create and set 2 new DWORD values (note that these values are case sensitive);

    * WarnLargeFileSize
      Don’t set this higher than 4090445042 (decimal) or f3cf3cf2 (hexadecimal)
    * MaxLargeFileSize
      Don’t set this higher than 4294967295 (decimal) or ffffffff (hexadecimal)

The first value is how many MB a user can write to a pst-file. The second value is how many MB the system can write to a pst-file. This difference has to be at least 5% since there is more written to a pst-file than just user data.