According to the FAQ of this document Image Optimization for Link Page Posts.
What is the minimum image size required to generate a link share post with a large image?
600 x 315px is the minimum size that we require. However, we recommend that you use images that are at least 1200 x 630px for the best display on retina and high-resolution screens.
What will happen if the image is below the minimum requirements?
If the image is smaller than 600 x 315px, we will render it at 154 x 154px or smaller on mobile or desktop.