Bootstrap class: .modal-lg

<!-- Button trigger modal -->
<button class="btn btn-primary" data-toggle="modal" data-target=".bd-example-modal-lg">
  Large modal
</button>

<!-- Modal -->
<div class="modal fade bd-example-modal-lg" tabindex="-1" role="dialog"
     aria-labelledby="myLargeModalLabel" aria-hidden="true">
  <div class="modal-dialog modal-lg">
    <div class="modal-content">
      ...
    </div>
  </div>
</div>

Preview

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