14-use-the-flex-basis-property-to-set-the-initial-size-of-an-item.html

<style>
  #box-container {
    display: flex;
    height: 500px;
  }

  #box-1 {
    background-color: dodgerblue;
    height: 200px;
    flex-basis: 10em;
  }

  #box-2 {
    background-color: orangered;
    height: 200px;
    flex-basis: 20em;
  }
</style>

<div id="box-container">
  <div id="box-1"></div>
  <div id="box-2"></div>
</div>