code: public function useScreenResolution($width_height)
      {
          list($width, $height) = explode('x', $width_height);
  
          $this->getSessionDriver()->resizeWindow((int) $width, (int) $height);
      }
docs: Open the page with specified resolution.
  
  @param string $width_height
  String that satisfy the condition "<WIDTH>x<HEIGHT>".
  
  @example
  Given I should use the "1280x800" resolution
  
  @Given /^(?:|I should )use the "([^"]*)" screen resolution$/