"The file "drive:/path_to_your_.pst/outlook.pst" has reached its maximum size. To reduceObviously you can not delete any messages because you can't add the data file itself. This case should be only for Outlook 2003 without sp2 because it limits .pst file size to somewhat 2GB, however I encountered a situation where this message was generated by Outlook 2007.
the amount of data in the file, select some items that you no longer need and
then permanently delete them by pressing SHIFT + DELETE".
Fastest workaround for this is to share the .pst file and add it to outlook 2007 on another computer in you local area network and delete some messages to lower the size of .pst file. That is if you have a network and another PC with outlook installed.
The other way is to download microsoft tool for shrinking .pst files, then fix the file with SCANPST.EXE ) (if you have outlook 2007 it will be located in "drive:\Program Files\Microsoft Office\OFFICE12") and then try adding the Data File again.