SEO plugin question

I’m using the SEO plugin to add all the header stuff. At the end of the docs it talks about setting up the image. It gives an example of 100*100. Is that the standard size for the “social” images or is there a range?

I added:

  - scope:
    path: ""
      image: /images/social/my-logo.png

to the_config.yml and I got an “undefined method ‘key?’ for nil:NilClass” and it could not read any of the posts. I’m using Jekyll v3.4.0 if that matters.