Before Outlook 2003, PST files used ANSI encoding which limited the size to 2Gb, and beyond 2Gb Outlook would not recognize the file. In Outlook 2003 and newer they reconfigured this to allow for a default maximum PST file size of 20Gb using Unicode encoding. The maximum file size can also be configured to be larger if needed.
The location of the archive file can be an issue as well, specifically in Outlook 2010 which will hang/crash when trying to open an archive file located across the network. There does not seem to be an issue with this in Outlook 2003 and 2007, though keep in mind if your archive file is very large, performance will be impacted when opening it, especially over the network.
It is always best practice to delete the emails you don't need and to clean out your Inbox and other folders of unneeded emails regularly.
Additionally if you have multiple archive files, determine if you need to have all of them open every time you use Outlook as this can affect performance as well.
Generally speaking (when using Outlook 2003 or newer), if your .pst file gets larger than a few Gb's it's a good idea to make an archive of it. When doing so there are options to archive emails older than a given date. Using this feature will allow you to archive specifically what you want. There are also AutoArchive settings which can be applied to your Inbox or even specific folders within your Inbox. For instance you could have emails older than 1 month automatically added to an archive for your Inbox or any other folder in Outlook.
Lastly, even though your Outlook 2003 and newer archive and .pst files can grow to 20Gb or larger, if you find either one of these files is larger than say 5-10Gb you may want to consider probing through them to see if there's anything you can get rid of...if not, then at least you know you can grow to at least 20Gb without any issues.
By Kyle Stewart