--- /dev/null
+@charset "UTF-8";
+
+/// Sets the `width` and `height` of the element in one statement.
+///
+/// @argument {number (with unit) | string} $width
+///
+/// @argument {number (with unit) | string} $height [$width]
+///
+/// @example scss
+/// .first-element {
+/// @include size(2em);
+/// }
+///
+/// // CSS Output
+/// .first-element {
+/// width: 2em;
+/// height: 2em;
+/// }
+///
+/// @example scss
+/// .second-element {
+/// @include size(auto, 10em);
+/// }
+///
+/// // CSS Output
+/// .second-element {
+/// width: auto;
+/// height: 10em;
+/// }
+///
+/// @require {function} _is-size
+
+@mixin size(
+ $width,
+ $height: $width
+) {
+ @if _is-size($height) {
+ height: $height;
+ } @else {
+ @error "`#{$height}` is not a valid length for the `$height` argument " +
+ "in the `size` mixin.";
+ }
+
+ @if _is-size($width) {
+ width: $width;
+ } @else {
+ @error "`#{$width}` is not a valid length for the `$width` argument " +
+ "in the `size` mixin.";
+ }
+}