Click Start, click Run, type Regedit, and then click OK.
In the left pane, expand following registry key:
For Outlook 2010
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\14.0\Outlook\PST
For Outlook 2007
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Outlook\PST
For Outlook 2003
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0\Outlook\PST
Modify:
MaxLargeFileSize and WarnLargeFileSize