The window size can be maximized, set or resized
To maximize the window
driver.manage().window().maximize();
To set the window size
Dimension d = new Dimension(400,600);
driver.manage().window().setSize(d);
Alternatively,
The window size can be reset using JavaScriptExecutor
((JavascriptExecutor)driver).executeScript("window.resizeTo(1024, 768)");