<div class="container" id="container">
  <div class="item one">1</div>
  <div class="item two">2</div>
  <div class="item three">3</div>
  <div class="item four">4</div>
  <div class="item five">5</div>
</div>
body {
  background-color: #F5F5F5;
  color: #555;
  font-size: 1.1em;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
}

.container {
  margin: 30px auto;
  max-width: 800px;
  padding: 20px;
  -webkit-display: -webkit-box;
  -webkit-display: -webkit-flex;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  background-color: #594255;
}

.item {
  background-color: #fff;
  height: 100px;
  margin: 1%;
  text-align: center;
  line-height: 100px;
  font-weight: bold;
  font-family: sans-serif;
  font-size: 5em;
  color: #594255;
  /* START HERE */

  -webkit-flex: 0 1 100px;
  -webkit-box-flex: 0;
  -ms-flex: 0 1 100px;
  flex: 0 1 100px;
}

.one {
  /* flex: ; */
}

.two {
  /* flex: ; */
}

.three {
  /* flex: ; */
}

.four {
  /* flex: ; */
}

.five {
  /* flex: ; */
}