Outlook

PST Files Sizes Limits
Microsoft Office Outlook 2007 and Microsoft Office Outlook 2003 have both a different format and a larger overall size limit for the personal folders (.pst) file than the .pst files that are in the earlier versions of Microsoft Outlook. In Outlook 2002 and earlier, the .pst files are in the American National Standards Institute (ANSI) format, and the overall size has a limit of 2 gigabytes (GB).
 * http://support.microsoft.com/kb/830336

By default, .pst files are in the Unicode format in Outlook 2007 and in Outlook 2003. Additionally, the overall size of the .pst files has a limit that is more than the 2-GB limit that is imposed by the ANSI .pst files. By default, the limit for a Unicode .pst file in Outlook 2007 and in Outlook 2003 is configured to be 20 GB.