When you see this validation warning: "Image size smaller than recommended size", on a blog post page, this is just a warning, so it does not negatively affect your SEO for this page, however, you would ideally want to fix this and you can do so easily. You simply need to upload a higher resolution “featured image” in Shopify for this blog post.  

Why this warning appears?

When Google shows AMP pages in search results, if you have a high resolution image, then Google can show a high resolution preview image of your blog post. You don't absolutely have to have an image for this blog post, but it helps Google by showing a large image in the search preview. So if you don't have an image, or if your image is too small, Google gives you a warning. It's not an error, but Google just notifies you that it's best to have an image there.

Recommended image size

According to Google: Images should be at least 1200 pixels wide and at least 800,000 pixels when you multiply the width by height.

How to resolve this warning

This means that uploading an image that is 1200 x 680, or larger, will work. Once again, you can upload this image in your Shopify "featured image" setting, for this blog post.

You can edit the featured image here, for example, in your Shopify Admin Panel.

Once you've updated the image, you may go back to Google Search Console and click "Validate fix" for this warning.

Do you absolutely need to resolve this warning?

No, you can leave this warning as-is, and this page will still be AMPed. This is not an error, but merely a warning. Nonetheless, it's often best to resolve all warnings for added peace of mind.