This somewhat ugly but useful thing is called a QR code. Over the past few years these have been used more and more. I work at Walmart (i hate it), and I see them on almost everything now. QR codes are basically a way to bring print and multimedia together for the same purpose. The idea is to scan this with your smartphone and it will take you to a link, video, picture, or a website. If you scan the one above, it will take you back to this very blog you are reading. As far as how small they can go and still be scanned
themobilists say about 3/4". They should be high contrast with the background. The longer the url of the link the more complex the QR code is therefore being harder to read. There should be a border around the code at least 1/8" wide.
Source: http://www.themobilists.com/2011/10/11/qr-code-best-practices-in-print/