You should use an Image Dimension setting of at least 1200 x 630 pixels for the best display on high resolution devices. At the minimum, you should use an Image Dimension setting of 600 x 315 pixels to display link page posts with larger images. If your image is smaller than 600 x 315 px, it will still display in the link page post, but the size will be much smaller. See Maximizing Distribution for Media Content on Facebook for more information.