Edit in JSFiddle

<!-- modal 구동 버튼 (trigger) -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#mySmallCenterModal">
  Small Center
</button>
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#my80sizeCenterModal">
  80%size Center
</button>

<!-- Small Modal at Center -->
<div class="modal modal-center fade" id="mySmallCenterModal" tabindex="-1" role="dialog" aria-labelledby="mySmallCenterModalLabel">
  <div class="modal-dialog modal-sm modal-center" 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 제목</h4>
      </div>
      <div class="modal-body">
        Modal 내용
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">닫기</button>
      </div>
    </div>
  </div>
</div>

<!-- 80%size Modal at Center -->
<div class="modal modal-center fade" id="my80sizeCenterModal" tabindex="-1" role="dialog" aria-labelledby="my80sizeCenterModalLabel">
  <div class="modal-dialog modal-80size modal-center" role="document">
    <div class="modal-content modal-80size">
      <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 제목</h4>
      </div>
      <div class="modal-body">
        Modal 내용
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">닫기</button>
      </div>
    </div>
  </div>
</div>

.modal-dialog.modal-80size {
  width: 80%;
  height: 80%;
  margin: 0;
  padding: 0;
}

.modal-content.modal-80size {
  height: auto;
  min-height: 80%;
}

.modal.modal-center {
  text-align: center;
}

@media screen and (min-width: 768px) {
  .modal.modal-center:before {
    display: inline-block;
    vertical-align: middle;
    content: " ";
    height: 100%;
  }
}

.modal-dialog.modal-center {
  display: inline-block;
  text-align: left;
  vertical-align: middle;
}