// Set `width` and `height` in a single statement

@mixin size($value) {
  $width: nth($value, 1);
  $height: $width;

  @if length($value) > 1 {
    $height: nth($value, 2);
  }

  @if is-size($height) {
    height: $height;
  }

  @else {
    @warn "`#{$height}` is not a valid length for the `$height` parameter in the `size` mixin.";
  }

  @if is-size($width) {
    width: $width;
  }

  @else {
    @warn "`#{$width}` is not a valid length for the `$width` parameter in the `size` mixin.";
  }
}

@charset "UTF-8";

/// Sets the `width` and `height` of the element.
///
/// @param {List} $size
///   A list of at most 2 size values.
///
///   If there is only a single value in `$size` it is used for both width and height. All units are supported.
///
/// @example scss - Usage
///   .first-element {
///     @include size(2em);
///   }
///
///   .second-element {
///     @include size(auto 10em);
///   }
///
/// @example css - CSS Output
///   .first-element {
///     width: 2em;
///     height: 2em;
///   }
///
///   .second-element {
///     width: auto;
///     height: 10em;
///   }
///
/// @todo Refactor in 5.0.0 to use a comma-separated argument

@mixin size($value) {
  $width: nth($value, 1);
  $height: $width;

  @if length($value) > 1 {
    $height: nth($value, 2);
  }

  @if is-size($height) {
    height: $height;
  }

  @else {
    @warn "`#{$height}` is not a valid length for the `$height` parameter in the `size` mixin.";
  }

  @if is-size($width) {
    width: $width;
  }

  @else {
    @warn "`#{$width}` is not a valid length for the `$width` parameter in the `size` mixin.";
  }
}

Add SassDoc for size mixin
Add SassDoc for size mixin
