Saas @mixin for change measures in responsive desing - http://jesus.perezpaz.es/2013/03/saas-mixin-for-change-measures-for-responsive-desing/

gistfile1.scss

$responsive_resize: 0.6;
$px:1px;

@mixin res($rpx) {
    $px: $rpx;
    @content;
    @media screen and (max-width: 320px) {
        $px: $rpx*$responsive_resize;
        @content;
    }
}

gistfile2.scss

.myclass {
  @include res(400px) { width: $px; }
}

gistfile3.css

.myclass {
  width: 400px; }
  @media screen and (max-width: 495px) {
    .myclass {
      width: 240px; } }

gistfile4.scss

.myclass {
  @include res(400px) { width: $px; }
  @include res(200px) { height: $px; }
}

gistfile5.css

.myclass {
  width: 400px;
  height: 200px; }
  @media screen and (max-width: 495px) {
    .myclass {
      width: 240px; } }
  @media screen and (max-width: 495px) {
    .myclass {
      height: 120px; } }