@mixin size($size) {

@if length($size) == 1 {
  @if $size == auto {
    width:  $size;
    height: $size;
  }

  @else if unitless($size) {
    width:  $size + px;
    height: $size + px;
  }

  @else if not(unitless($size)) {
    width:  $size;
    height: $size;
  }
}

// Width x Height
@if length($size) == 2 {
  $width:  nth($size, 1);
  $height: nth($size, 2);

  @if $width == auto {
    width: $width;
  }
  @else if not(unitless($width)) {
    width: $width;
  }
  @else if unitless($width) {
    width: $width + px;
  }

  @if $height == auto {
    height: $height;
  }
  @else if not(unitless($height)) {
    height: $height;
  }
  @else if unitless($height) {
    height: $height + px;
  }
}

}