Edit in JSFiddle

<!-- modal 구동 버튼 (trigger) -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#myFullsizeModal">
  Fullsize Modal
</button>
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#my80sizeModal">
  80%size Modal
</button>

<!-- Fullsize Modal -->
<div class="modal fade" id="myFullsizeModal" tabindex="-1" role="dialog" aria-labelledby="myFullsizeModalLabel">
  <div class="modal-dialog modal-fullsize" role="document">
    <div class="modal-content modal-fullsize">
      <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">전지 Modal</h4>
      </div>
      <div class="modal-body">
        화면을 가득 채우므로 가장자리가 각져있습니다.
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">닫기</button>
      </div>
    </div>
  </div>
</div>

<!-- 80% size Modal -->
<div class="modal fade" id="my80sizeModal" tabindex="-1" role="dialog" aria-labelledby="my80sizeModalLabel">
  <div class="modal-dialog modal-80size" 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">조금 부족한 Modal</h4>
      </div>
      <div class="modal-body">
        부족한 만큼 둥근 마음을 갖고 있습니다.
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">닫기</button>
      </div>
    </div>
  </div>
</div>

.modal-dialog.modal-fullsize {
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
}

.modal-content.modal-fullsize {
  height: auto;
  min-height: 100%;
  border-radius: 0;
}

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

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