FirefoxOptions firefoxOptions = new FirefoxOptions();
  firefoxOptions.AddArguments("--headless");
  firefoxOptions.AddArguments("--window-size=1920,1080");

  FirefoxDriver firefoxDriver = new FirefoxDriver(firefoxOptions);

  firefoxDriver.Manage().Window.Maximize();

  firefoxDriver.Navigate().GoToUrl("https://www.google.com/");
  var size = firefoxDriver.Manage().Window.Size; //{Width = 1366 Height = 768}