Sloth
NO, I win! Mine goes to 4144 characters!

alias passgen {
  var %r
  while $len(%r) < $1 {
    %r = %r $+ $chr($remove($r(48,122),58,59,60,61,62,63,64,91,92,93,94,95,96))
  }
  return %r
}

However, I like Thelmac's best but to be perfectly 'fair' his should be like this:

alias genpass { return $eval($ $+ +( $str($!iif($floor($calc($r(0,61)/10)),$iif($r(0,1),$r(A,Z),$r(a,z)),$r(0,9)) $+ $chr(44),$1) ),2) }

This way is a close enough approximation:

alias genpass { return $eval($ $+ +( $str($!iif($r(0,5),$iif($r(0,1),$r(A,Z),$r(a,z)),$r(0,9)) $+ $chr(44),$1) ),2) }

His original version produces about 50% numbers in the string.

Also, Thelmac if you need a password 469 characters you can:
$str($genpass(67),7)

alias PWgen $left($str($genpass(79),52), $1)

To produce up to 4108 character passwords.