Resize Capybara window

I'm trying to resize a browser window before my tests in Capybara.

I've used this solution: http://ift.tt/1TLG0ut

Which should work, but I am getting this error:

NoMethodError:
   undefined method `manage' for #<Capybara::RackTest::Browser:0x005555fdbe4210>

Are there any alternatives to this method or any solutions to manage not being available?