Edit in JSFiddle

.square {
  width: 100px;
  height: 100px;
  background: #439AE2;
  margin: 20px;
  border-radius: 5px;
  -webkit-transition: width 1s;
  -moz-transition: width 1s;
  -o-transition: width 1s;
  transition: width 1s;
}

.square:hover {
  width: 250px;
  cursor: pointer;
}

<div class="square"></div>