Capybara view as iphone helper. Change the user agent string to be whatever device you'd like. It's also possible to add view-port dimensions.

mobile_capybara.rb

# put this in `features/support/env.rb`
Capybara.register_driver :chrome do |app|
  args = []
  args << "--user-agent='Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile    /9A334 Safari/7534.48.3'"
  Capybara::Selenium::Driver.new(app, {:browser => :chrome, :args => args})
end

Before('@mobile') do
  Capybara.current_driver = :chrome
end

After('@mobile') do
  Capybara.use_default_driver
end

# or you can set it in a step
# `features/step_definitions/mobile_steps.rb`
Given /^I use a mobile device$/ do
  Capybara.current_driver = :chrome
end