Who would have thought sharing an image on social media could be so complicated. After all the particulars, it appears that it boils down to using 1024×512 for horizontal images, and 800×1200 for vertical images.
It surely helps to scale and crop your images to the perfect size for each social network, but the bottom line is that if you share compelling images (or pictures of Kim Kardashian’s ass), people will click and open the full size image anyway.