<div class="container"><div class="widthFixed"></div><div class="widthNoFixed"></div></div>

.container{
  max-width: 1000px;
  margin: 0 auto;
}

.widthFixed{
  width: 338px;
  height: 235px;
  background: red;
  display: inline-block;
}

.widthNoFixed{
  display: inline-block;
  max-width: 400px;
  height:235px;
  background:green;
  vertical-align: top;
}