Edit in JSFiddle

<div class="flex-container">
  <div class="greenbox">Some arbitrary content<br> that will set the dimensions of the div</div>
</div>

.flex-container{
  width: 100%;
  height: 100vh;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  -webkit-box-pack: center;
      -ms-flex-pack: center;
          justify-content: center;
}

.greenbox {
  background: limegreen;
  text-align: center;
 padding: 2em;
}