I am searching a lot but not able to find the minimum heightxwidth required for facebook share link to show thumbnail.
I tried with my website's logo but it didn't work, then I tried with bigger image and it worked but what is the minimum size required?
In the docs about the feed dialog you have this information:
(...)The URL of a picture attached to this post. The picture must be at least 200px by 200px. See our documentation on maximizing distribution for media content for more information on sizes.
Here you have official info about Sharing Best Practices, and you can get more info about other facebook sizes here