set the width and heigth of the browser and then check if the width of navigation bar has full length by executing jQuery.

amazon_set_window_size_and_check_css.rb

require 'capybara/dsl'
include Capybara::DSL
Capybara.default_driver = :selenium


visit "http://www.amazon.com/"

# the screen size will be fixed as the following resolution
# then navigation bar is expected to fill full size of header.
# if there is a scroll bar, its width shoul be subtracted as 15px. 
screen_sizes = [[640, 480], [1024, 768], [1280, 800], [1368, 768], [1440, 900], [2560, 1440]]
screen_sizes.each do |x, y|
	page.driver.browser.manage.window.resize_to(x, y)
	top_bar_width = page.evaluate_script("jQuery('#nav-bar-outer').css('width')")
	puts "Width of navigation bar: #{top_bar_width}\n"
	puts "Width of window: #{x} - #{x-15}\n"
	puts "\n-------------------------\n"
	[x, x-15].include? top_bar_width
end