Have you tried setSize()
method from WebDriver.Window
? Here is the documentation.
// untested Java code, only provides the logic
// please debug and refer to the documentation
import org.openqa.selenium.Dimension;
WebDriver driver = new PhantomJSDriver();
driver.manage().window().setSize(new Dimension(1920, 1080));
// or driver.manage().window().maximize();
driver.get("http://stackoverflow.com/questions/21743350/how-to-set-screen-window-size-when-using-ghostdriver");