Hi @Mbrinton, the article that @Colan Nielsen is spot on. However, I would add that there are some exceptions to the best practice of using square images. Please see my post regarding variations of image sizes that could give you a leg up on your competition: Full Size picture showing up on mobile SERP
A brief update, for most of the search terms I mentioned in the original post thread to replicate the full-width images or carousel images, they still work on my Samsung phone but not on Google's Anonymous Ad Tool. I'd suggest trying your specific search terms on your phone and check if results with full-size images show up. If so, then use a full width image (e.g. 16:9) along with the proper page markup and see what happens. We are primarily a Wordpress shop, so like Colan's referenced article mentions, the easiest is to use Yoast or Rankmath to set the featured image without the need for manual markup. I will say we have the best luck by using the actual image in the body of the page that is also set as the featured image.
BTW, I've yet to see a full-width image on desktop, so if you choose to go the full-width approach I'd suggest using an image that will look appealing if the image is center cropped in a square shape. However, being that at least 65% of searches are done on mobile, I feel the benefit of having a full-width image is pretty significant.