Its difficult to take screenshot of whole web page with the default print screen option. So here is a service known as Super Screenshot with which you can take screenshot of the whole web page with a single click on a button.
Just enter the URL of the website whose screenshot you want and click on GO and you are done. Not only this you will get alot of options in the right sidebar asking about image format, size and dimension.
See the Screen Shot:
visit and Screenshot the full page: http://www.superscreenshot.com/