When using Behat with Mink and PhantomJS (in WebDriver mode), control over the window size can be very important when working with responsive design.
The Mink documentation states that if you use resizeWindow() without filling in the third argument (window name), it will use the current window.
This works for Selenium where calling '/window/size' works on the main window, but not for PhantomJS. Instead, use the window name 'current'.
$this->getSession()->resizeWindow(1440, 900, 'current');