Bourbon Fundamentals - Introduction & Size Mixin

This topic is for the [Introduction & Size Mixin][] exercise in the [Bourbon Fundamentals][] trail. Post any questions, corrections, or pointers you have to share with other Upcase subscribers.
[Introduction & Size Mixin]: https://exercises.upcase.com/exercises/introduction-to-bourbon
[Bourbon Fundamentals]: https://thoughtbot.com/upcase/bourbon-fundamentals