Although documented in the manual, getfilesize is limited to files under 2gb. It is not that unusual to have files this big or bigger now days.

The following thread brought up the issue, and its obviously possible using FSO.
http://www.kixtart.org/forums/ubbthreads.php?ubb=showflat&Number=204979#Post204979

Ruud, is this something that could be easily fixed in Kix?