def [](new_size)
      if new_size.is_a? Directive
        tags.merge! new_size.tags_for_sized_directive
      else
        tags[:size] = new_size
      end

      self
    end
def [](new_size)
      if new_size.is_a? Directive
        tags.merge! new_size.tags_for_sized_directive
      else
        tags[:size] = new_size
      end

      self
    end
Changes the size of the directive to the given size.  It is
 possible for the given value to the a directive; if it is,
 it just uses the name of the directive.

 @param new_size [Numeric, Directive]
 @return [self]