Place gravatar.rb into your site's _plugins directory

Just use {{ 'me@example.com' | gravatar }}. It'll return the link to your Gravatar.

<figure>
  <img src="{{ 'me@example.com' | gravatar }}" alt="My beautiful face.">
</figure>

Go into gravatar.rb and edit line 6. Change ?s=600 to ?s=[your-size-here]. If your Gravatar is a high-res image, you can use a relatively large size. If it's a low-quality image, do whatever you want. I don't care.