This worked for me

.modal-dialog,
.modal-content {
    /* 80% of window height */
    height: 80%;
}

.modal-body {
    /* 100% = dialog height, 120px = header + footer */
    max-height: calc(100% - 120px);
    overflow-y: scroll;
}

Fiddle: http://jsfiddle.net/mehmetatas/18dpgqpb/2/

The .modal-content block is inside the .modal-dialog block. Hence, tell the child to be of the same height as the parent. Try:

.modal-dialog {
  height: 90%; /* = 90% of the .modal-backdrop block = %90 of the screen */
}
.modal-content {
  height: 100%; /* = 100% of the .modal-dialog block */
}

Check:

@import url('https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css');

.modal-tall .modal-dialog {
  height: 90%;
}
.modal-tall .modal-content {
  height: 100%;
}

/* fix SO stick navigation ;) */
@media (min-width: 768px) { body { padding-top: 75px; } }

<div class="container">
  <button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#modalDefault">
    Default modal
  </button>

  <button type="button" class="btn btn-warning btn-lg" data-toggle="modal" data-target="#modalTall">
    Tall modal
  </button>
</div>

<div id="modalDefault" class="modal fade" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">Default modal</h4>
      </div>
    </div>
  </div>
</div>

<div id="modalTall" class="modal modal-tall fade" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">Modal has 90% height</h4>
      </div>
    </div>
  </div>
</div>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>

I caved in and wrote coffeescript to fix this. If anyone's interested, here's the coffeescript:

fit_modal_body = (modal) ->
  header = $(".modal-header", modal)
  body = $(".modal-body", modal)

  modalheight = parseInt(modal.css("height"))
  headerheight = parseInt(header.css("height")) + parseInt(header.css("padding-top")) + parseInt(header.css("padding-bottom"))
  bodypaddings = parseInt(body.css("padding-top")) + parseInt(body.css("padding-bottom"))

  height = modalheight - headerheight - bodypaddings - 5 # fudge factor

  body.css("max-height", "#{height}px")

# Here you need to bind your event with the appropriate modal, as an example:
$(window).resize(() -> fit_modal_body($(".modal")))

Or the equivalent javascript as generated per above.

var fit_modal_body;

fit_modal_body = function(modal) {
  var body, bodypaddings, header, headerheight, height, modalheight;
  header = $(".modal-header", modal);
  body = $(".modal-body", modal);
  modalheight = parseInt(modal.css("height"));
  headerheight = parseInt(header.css("height")) + parseInt(header.css("padding-top")) + parseInt(header.css("padding-bottom"));
  bodypaddings = parseInt(body.css("padding-top")) + parseInt(body.css("padding-bottom"));
  height = modalheight - headerheight - bodypaddings - 5;
  return body.css("max-height", "" + height + "px");
};

$(window).resize(function() {
  return fit_modal_body($(".modal"));
});

Pure CSS solution, using calc

.modal-body {
    max-height: calc(100vh - 200px);
    overflow-y: auto;
}

200px may be adjusted in accordance to height of header & footer

In your code, for the modal-dialog div, add another class, modal-lg:

<div class="modal-dialog modal-lg">

Or if you wanna centre your modal dialog, use:

.modal-ku {
  width: 750px;
  margin: auto;
}

If you want the modal to consume 90% of the viewport height, you don't need any JS. CSS has a special unit for that called vh. 100vh is 100% of the viewport.

In your case, you'll just need:

.modal {
  height: 90vh;
}

You can also change it to max-height if necessary.

Try with this js to calculate the modal height:

$(document).ready(function() {
    $(".showcomments").click(function() {
        $('#myModal').modal('show');
        rescale();
    });
    $("[rel=tooltip]").tooltip();
});
function rescale(){
    var size = {width: $(window).width() , height: $(window).height() }
    /*CALCULATE SIZE*/
    var offset = 20;
    var offsetBody = 150;
    $('#myModal').css('height', size.height - offset );
    $('.modal-body').css('height', size.height - (offset + offsetBody));
    $('#myModal').css('top', 0);
}
$(window).bind("resize", rescale);

Test: http://jsfiddle.net/fUZxA/23/