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How to configure the size limit for both (.pst) and (.ost) files in Outlook

Microsoft Outlook 2016, Outlook 2013, Outlook 2010, Outlook 2007, and
Outlook 2003 support American National Standards Institute (ANSI) and
UNICODE personal folders (.pst) and offline folder (.ost) files. This
article describes how to use the following four registry entries to limit
the size of both the .pst and the .ost files:

- The MaxFileSize registry entry
- The WarnFileSize registry entry
- The MaxLargeFileSize registry entry
- The WarnLargeFileSize registry entry

Note: The WarnLargeFileSize and WarnFileSize registry entries do not enable
Outlook to warn you before the file size limit is reached.

Note: The settings that are discussed in this article apply to both .ost and
.pst files. If you modify these registry values, this can affect .ost files
that are used with Cached Exchange Mode, with AutoArchive, and with .pst.
files. If Outlook is configured to download shared folders, the contents of
shared folders are stored in the local Offline Outlook Data (.ost) file. If
the shared folders contain many items or large attachments, the size of the
.ost file may grow significantly. Additionally, Outlook 2013 introduced
support for Site Mailboxes. If an Outlook 2013 client is working in a
Microsoft Exchange Server 2013/Microsoft SharePoint 2013 environment and is
granted permission to a site mailbox, the site mailbox is automatically
added to the Outlook 2013 profile. If Download Shared Folders is enabled,
the site mailbox contents are synchronized to the local .ost file. This can
result in the .ost file exceeding the set limit.

- Click Start, and then click Run.
- In the Open box, type regedit, and then click OK.
- In the left pane, expand My Computer, and then expand HKEY_CURRENT_USER.
- Expand Software, and then expand Policies.
- Expand Microsoft, and then expand Office.

    Expand:
    11.0 for Outlook 2003    12.0 for Outlook 2007
    14.0 for Outlook 2010    15.0 for Outlook 2013
    16.0 for Outlook 2016

    Then expand Outlook.

- Click PST, and then right-click MaxFileSize in the right pane.
- Click Modify, and then type the value in the Value data box.
- Click OK.
- Right-click WarnFileSize, and repeat steps 8 through 9.
- Right-click MaxLargeFileSize, and repeat steps 8 through 9.
- Right-click WarnLargeFileSize, and repeat steps 8 through 9.

Note: You may have to create the registry values if they do not exist. All
types are DWORD (32bit)
Note: All values are in bytes.
Note: 1GB=1*1024*1024*1024byte; 1MB=1*1024*1024byte; 1KB=1*1024byte.

It is recommended that the values between the MaxFileSize registry entry and
the WarnFileSize registry entry, and the values between the MaxLargeFileSize
registry entry and the WarnLargeFileSize registry entry be at least 5
percent (%) so that internal processes are not hindered from continuing.

If the value of the MaxFileSize registry entry ever exceeds the ANSI 2
gigabyte (GB) limit on either the .pst or the .ost files, the value will be
ignored to limit the size to 2 GB to prevent corruption. The default value
for the WarnFileSizeregistry registry entry is calculated to be 95% of the
MaxFileSize registry entry for a UNICODE file, and it remains at
1,950,368,768 bytes for small ANSI files.

Note: You can set the UNICODE limits beyond the values that are listed in
the table. However, we do not recommend doing this because performance can
decrease.

If .ost files or .pst files reach the limit that is specified in the
WarnFileSize or the WarnLargeFileSize registries, the compaction function is
triggered to try to reduce the size of the file to a usable level. When the
WarnFileSize or the WarnLargeFileSize limit is reached, e-mail messages
cannot be sent (provided that sent e-mail messages are stored in the Sent
Items folder), and items cannot be copied or moved within the file. If the
file is an archive .pst file that is used for AutoArchive, the AutoArchive
operation will fail. However, e-mail messages can be deleted or archived
from a .pst or from an .ost file that is currently being used as the default
delivery location.

https://support.microsoft.com/en-us/topic/how-to-configure-the-size-limit-for-
both-pst-and-ost-files-in-outlook-2f13f558-d40e-9c2a-e3b6-02806fa535f4