This should be much much faster as it uses $findfiles internal looping ability.

Code:
alias biggestfile {
  set -u %largefile
  set -u %largefilesize 0
  !.echo -q $findfile($1,*.*,0,checkfile $1-)
  return $iif($prop == size,$bytes(%largefilesize,k3),%largefile)
}
alias checkfile {
  if ($file($1-) > %largefilesize) {
    %largefile = $1-
    %largefilesize = $v1
  }
}


$biggestfile(dir) returns the name of the largest file, $biggestfile(dir).size returns the size of the largest file.