To recover a very large Microsoft Outlook PST file (more than 4 GB), you'll need to have Recovery Toolbox for Outlook installed on your computer, as well as the following:
If this software is installed, Recovery Toolbox for Outlook won't be limited to the size of operating data that 32-bit systems have. The program will be able to completely restore and save the data in a new PST file when working with the 64-bit versions of Microsoft Outlook and Microsoft Windows. If Microsoft Outlook 32-bit is installed on the computer, Recovery Toolbox for Outlook will encounter a limitation due to the amount of RAM that can be used in the restoration of data.
If you have the installation disc or distribution kit, you can install Microsoft Outlook 64-bit along with the 32-bit version of Microsoft Outlook. You'll find instructions for the installation of Microsoft Outlook 64-bit here.