[Zope] displaying FileSize in KB or MB in stead of bytes (newbie)

When I call the '&dtml-size' or '<dtml-var (get_)size>' the bytesize is displayed. When using the ZMI U get a MB or KB recalculation of the filesize. Can I call this recalculation from within a document? Or do I have to write a script myself? TIA Roel.