Never mind, I found it in the plugin settings and it was checked by default. I also have the Add Open Graph Tags box checked and they are being correctly generated for the posts, including a link to the proper image that I’d like to be displayed on Facebook, and the image is 1880×1028, fulfilling the size requirements for the large thumbnails. Lastly, my link attachment type is set to shared. All of the criteria for the large thumbnails are met and yet I still get small thumbnails in the Newsfeed to the post.
