Full width flex item container

If you want the parent container to stretch to the full width of the page, you can omit a width and height value for the parent, but you will need to provide a height for the flex-item children:

.flex-container-row {
    display: flex;
    flex-direction: row;
}
...

<div class="flex-container-row">
  <amp-img src="img/placeholder.png" layout="flex-item" height="200">
  </amp-img>
  <amp-img src="img/placeholder.png" layout="flex-item" height="100">
  </amp-img>
</div>