Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.3)
// Compass (v1.0.0.alpha.18)
// ----

// Map storing sizes when the mixin is being called
$sizes: (
  width  : (),
  height : (),
);

// A mixin defining both the width and the height
// ---
// The main problem with this mixin is it doesn't try to 
// merge selectors when there is either the width or the height
// repeated from one call to the other
// ---
// So here is an over-engineered version which use placeholders
// to group selectors when a width or height has already been used
// ---
// @param: [number] $width: width
// @param: [number] $height ($width): height
// ---
@mixin size($width, $height: $width) {
  $dimensions: (width $width, height $height);

  // If the size has not been defined yet
  // Add the value to the map
  // And create a placeholder at root
  @each $dimension, $dimension-value in $dimensions {
    @if not index(map-get($sizes, $dimension), $dimension-value) {
      $sizes: map-merge($sizes, ($dimension: append(map-get($sizes, $dimension), $dimension-value)));
      @at-root %#{$dimension}-#{$dimension-value} {
        #{$dimension}: $dimension-value;
      }
    }
    
    // Extend the placeholder
    @extend %#{$dimension}-#{$dimension-value};
  }
}

.a {
  @include size(10px, 20px);
}

.b {
  @include size(10px, 2em);
}

.c {
  @include size(1px, 2em);
}

SassMeister-output.css

.a, .b {
  width: 10px;
}
.a {
  height: 20px;
}

.b, .c {
  height: 2em;
}

.c {
  width: 1px;
}