I would just hide the items greater then the number you want show like this jsfiddle

** Edit **

I modified the the above fiddle

here is the html

<select name="foo" id="myselect" multiple="true" siz="4">
    <option>1</option>
    <option>2</option>
    <option>3</option>
    <option>4</option>
    <option>5</option>
    <option>6</option>
</select>
<input type="checkbox" id="mycheckbox" />

and the jQuery

   $('#myselect').data('removedOptions', $('#myselect > option:gt(3)').detach() );

I have stored the options so you can put them back like this

var $dropdown = $('#myselect');
  $dropdown.prepend( $dropdown.data('removedOptions') );

@mcgrailm: Your solution doesn't work on Chrome. So, I suggest we can using code with remove() function replace for hide() function.

Here is another version of @mcgrailm: http://jsfiddle.net/vietean/W4nzn/1/