This Behat definition resizes the browser. I've discovered that Chrome defaults to a fairly wide browser whereas PhantomJS defaults to a very narrow browser... Either way, it's important to be able to resize the browser window so you can get specific versions of your sites/menus *on purpose* rather than by accident!

browserResizeDefinition.php

// Include the following step definition in your FeatureContext.php file:

/**
* @Given /^I set browser window size to "([^"]*)" x "([^"]*)"$/
*/
public function iSetBrowserWindowSizeToX($width, $height)
{
  $this->getSession()->resizeWindow((int)$width, (int)$height, 'current');
}

browserResizeStep.feature

# Include the following step in your .feature file, editing the sizes as appropriate:

Given I set browser window size to "1024" x "768"