PSTs created in Outlook 2002 and earlier versions are saved in ANSI format, which has a 2.1GB limit. Outlook 2003 and later offer both ANSI and Unicode formats for PST creation. Unicode PSTs have a theoretical 36TB limit which makes them a better choice, providing that backward compatibility is not an issue.
So how can you tell if a PST is in ANSI or Unicode format?
One way is to download a free utility called ListPSTs from http://www.maclean.com. You run this utility from the command line against the file or folder that contains the PST(s). The output displays the format of the PST files, as shown above.
Another way to tell without having to use a separate utility is by viewing the properties of the PST from within Outlook, itself. When you add the PST to Outlook, pay attention to the Format field of the PST, as shown below:
Unicode formatted PSTs will display the format, "Personal Folders File". ANSI formatted PSTs will display the format, "Personal Folders File (Outlook 97-2002)".