code: public void setVariation(DeviceType device, DeviceVariation variation, String desiredSDKVersion)
        throws WebDriverException {
      deviceName = AppleMagicString.getSimulateDeviceValue(device, variation, desiredSDKVersion, instrumentsVersion);
      setDefaultSimulatorPreference("SimulateDevice", deviceName);
  
      String subfolder = getSDKSubfolder(desiredSDKVersion);
      File sdk = new File(info.getXCodeInstall(), subfolder);
  
      if (!sdk.exists()) {
        throw new WebDriverException(
            "Cannot point simulator to requested sdk version " + sdk.getAbsolutePath());
      }
      setDefaultSimulatorPreference("currentSDKRoot", sdk.getAbsolutePath());
    }
docs: update the preference to have the simulator start in the correct mode ( ie retina vs normal,
  iphone screen size ).