@mixin size($size) {

$height: nth($size, 1);
$width: $height;
@if length($size) > 1 {
  $height: nth($size, 2);
}
@if $height == auto or (type-of($height) == number and not unitless($height)) {
  height: $height;
}
@if $width == auto or (type-of($width) == number and not unitless($width)) {
  width: $width;
}

}