'Override `ObjectStore`\'s stub by return file size on disk.
Returns 0 if the object doesn\'t exist yet or other error.'

def size(self, obj, **kwargs):

    if self.exists(obj, **kwargs):
        try:
            return os.path.getsize(self.get_filename(obj, **kwargs))
        except OSError:
            return 0
    else:
        return 0
