EDIT: The current best practices regarding Open Graph image sizes are officially outlined here: https://developers.facebook.com/docs/sharing/best-practices#images
There was a post in the Facebook developers group today, where one of the FB guys uploaded a PDF containing their new rules about image sizes – since that seems to be available only if you’re a member of the group, I uploaded it here: http://www.sendspace.com/file/ghqwhr
And they also said they will post about it in the developer blog in the coming days, so keep checking there as well
To summarize the linked document:
- Minimum size in pixels is 600×315
- Recommended size is 1200×630 – Images this size will get a larger display treatment.
- Aspect ratio should be 1.91:1