<div class="wrap">
  <div class="child">
    I'm am element in block. I have the very long content in me. My role is fill child element to make his has a unfixed height.
  </div>
</div>

body {
  background: #f06d06;
  padding-top: 40px;
}
.wrap {
  position: relative;
  background: #fff;
  border-radius: 5px;
  width: 300px;
  height: 300px;
  margin: 0 auto;
  text-align: center;
}
.child {
  border: 1px solid #eee;
  border-radius: 5px;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 150px;
  color: #666;
}