<h1>Bootstrap 3 Variable-Width Modal</h1>
<p>I wanted to override the default Bootstrap modal so it's variable-width, and height-optimized.</p>
<p>Add "modal-wide" to the main modal div, and adjust the width in the CSS. In this example, I'm using 90%.</p>
<p>Since I'm using jQuery to set the max-height of the content area based on the browser dimensions, the modal will be only as tall as necessary, and will provide a scrollbar if needed.</p>

<a data-toggle="modal" href="#normalModal" class="btn btn-default">Normal</a>

<a data-toggle="modal" href="#tallModal" class="btn btn-primary">Wide, Tall Content</a>

<a data-toggle="modal" href="#shortModal" class="btn btn-primary">Wide, Short Content</a>

<div id="normalModal" class="modal fade">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
        <h4 class="modal-title">Modal title</h4>
      </div>
      <div class="modal-body">
        <p>One fine body&hellip;</p>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div><!-- /.modal-content -->
  </div><!-- /.modal-dialog -->
</div><!-- /.modal -->

<div id="tallModal" class="modal modal-wide fade">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
        <h4 class="modal-title">Modal title</h4>
      </div>
      <div class="modal-body">
        <p>One fine body&hellip;</p>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div><!-- /.modal-content -->
  </div><!-- /.modal-dialog -->
</div><!-- /.modal -->

<div id="shortModal" class="modal modal-wide fade">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
        <h4 class="modal-title">Modal title</h4>
      </div>
      <div class="modal-body">
        <p>One fine body&hellip;</p>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <button type="button" class="btn btn-primary">Save changes</button>
      </div>
    </div><!-- /.modal-content -->
  </div><!-- /.modal-dialog -->
</div><!-- /.modal -->
.modal.modal-wide .modal-dialog {
  width: 90%;
}
.modal-wide .modal-body {
  overflow-y: auto;
}

/* irrelevant styling */
body { text-align: center; }
body p { 
  max-width: 400px; 
  margin: 20px auto; 
}
#tallModal .modal-body p { margin-bottom: 900px }
// when .modal-wide opened, set content-body height based on browser height; 200 is appx height of modal padding, modal title and button bar

$(".modal-wide").on("show.bs.modal", function() {
  var height = $(window).height() - 200;
  $(this).find(".modal-body").css("max-height", height);
});
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External CSS

  1. https://netdna.bootstrapcdn.com/bootstrap/3.0.0/css/bootstrap.min.css

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js
  2. https://netdna.bootstrapcdn.com/bootstrap/3.0.0/js/bootstrap.min.js