/// Set width and height in one line.
/// If height is unspecified, height will be set to
/// the same value as `$width` (effectively making this
/// a "square" function)
/// @param {Number} $width [100%]
/// @param {Number} $height [$width]
@mixin o-size($width: 100%, $height: $width) {
  width: $width;
  height: $height;
}

/// Symantic alias for `o-size` with only one argument
/// @param {Number} $size single value for both width and height
@mixin o-square($size) {
  @include o-size($size);
}