A mailbox of that size will not export in a single pass.
EXMERGE on Exchange 2003 supports a maximum of 2gb, and the export will be up to three times bigger than you are seeing in Exchange 2003. In fact it will be even larger because Exchange 2003 doesn't accurately report the mailbox size.
Therefore you will need to do an exmerge of the content, with filters to control the size - probably by year, and have multiple files. You need to watch out though - if the PST file goes above 2gb you will be unable to open it and you will need to repeat the export.
Exmerge copies the content out, it does not remove it.
When you are using exmerge, make sure that you include the recoverable items.
Being picky though, if the end user is three days away, then they have probably removed the bulk of the content already (unless they are very stupid). The export should have been done when they first handed their notice in and then done at regular intervals.
http://support.microsoft.com/kb/268754
And create a pst