Size mixin

Created by: MartijnCuppens

The size mixin is not documented in the docs. https://github.com/twbs/bootstrap/blob/29f178d89129732d35ae812667709c64cdb28a6f/scss/mixins/_size.scss#L1-L6

Shall we:

• document this
• or remove the mixin (width and height are not the most complex properties to remember)