pfgecr firefoxdriver set size window
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}