Hi cspringett,
DAT files should not be deleted manually. Deleting any of these files will lead to database corruption. I would rather delete the databases that are not used any more from openLCA.
If the storage size is really an issue for you, there is a solution that we are going to make public soon: libraries. Libraries provide the ability to re-use a background model for multiple database. That way, you won't have to duplicate the ecoinvent database multiple time.
So keep an eye on openLCA updates! ;)
Regards,
François