.width-transition {
  width: 100px;
  height: 50px;
  background: blue;
  transition: width 2s;
  -moz-transition: width 2s; /* Firefox 4 */
  -webkit-transition: width 2s; /* Safari and Chrome */
  -o-transition: width 2s;
}

.width-transition:hover {
  width: 300px;
}