<div class="box"></div>

html, body {
  height: 100%;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  background: #333;
  font-family: 'Lato', sans-serif;
}

.box {
  width: 150px;
  height: 150px;
  background-color: red;
  transition: all 1s;
}

.box:hover {
  width: 300px;
  background-color: orange;
}