Please see in the manual:
Exist() - http://www.scriptlogic.com/Kixtart/htmlhelp/Functions/exist.htm
GetFileSize() - http://www.scriptlogic.com/Kixtart/htmlhelp/Functions/getfilesize.htm
Del - http://www.scriptlogic.com/Kixtart/htmlhelp/Commands/del.htm

pseudo code;
 Code:
$filename="path\file"
$maxsizeinK=5
if exist($filename)
  if getfilesize($filename)>$maxsizeinK*1024
    del $filename
  endif
endif
_________________________
(... better days ahead)