minus-square and plus-square icon variants
Can there be small variants of the current `minus-square` and `plus-square` icons? The current ones are 18px in a 24px box. Can the smaller variants be 14px in a 24px box like the checkboxes?

![boxicons minus checkbox](https://user-images.githubusercontent.com/37782885/104060861-d78c4c00-51f7-11eb-8f74-bcf14f73ac6e.png)
