You cannot have an unlimited file size limit for .ost files in Outlook 2007 when you limit the .pst file size

On the computer that is running Microsoft Office Outlook 2007, you set the following registry subkey values to limit the file size of personal folders that have a .pst extension:

Registry subkeyValues

• MaxLargeFileSize
• WarnLargeFileSize

These changes are based on the following Microsoft Knowledge Base article: After you make these changes, individual offline folder files that have an .ost extension are also limited in file size, and the limit is equal to the MaxLargeFileSize registry entry in megabytes. Therefore, the offline folder files are no longer unlimited in file size.

How to obtain the hotfix

This issue is fixed in the Outlook 2007 post-Service Pack 1 Hotfix Package that is dated January 28, 2008. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

How to control the size of the .pst and .ost files

After you install the hotfix, you can independently control the size of the .pst files and of the .ost files. Set the MaxLargeFileSize DWORD to limit the .ost file size limit. Set the WarnLargeFileSize DWORD to limit the .pst file size limit. Use values within the following ranges:

Name: MaxLargeFileSize
Type: REG_DWORD
Range: 0x00000001 – 0x00005000
Default: 0x00005000 20,480 (20 GB)

Name: WarnLargeFileSize
Type: REG_DWORD
Range: 0x00000000 – 0x00005000
Default: 0x00004C00 19,456 (19 GB)

To limit the file sizes, follow these steps:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following subkey in the registry:
   HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Outlook\PST
3. Click MaxLargeFileSize.
4. On the Edit menu, point to New, and then click DWORD Value.
5. Type the value for the maximum .ost file size, and then click OK.
6. Click WarnLargeFileSize.
7. On the Edit menu, point to New, and then click DWORD Value.
8. Type the value for the maximum .pst file size, and then click OK.
9. Exit Registry Editor.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.