To increase the maximum size of pst-files and ost-files in Outlook, you’ll need to create and set 2 values in the following location in the Registry;

  • Outlook 2003
  • Outlook 2007
  • Outlook 2010
  • Outlook 2013
  • Outlook 2016 / Outlook 2019 / Microsoft 365

The following 2 new DWORD values must be created or set;

  • 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 or ost-file before receiving a warning that the file is full. The second value is how many MB the system can write to a pst-file or ost-file. This difference has to be at least 5% since there is more written to a pst-file and ost-file than just user data.