The rule of thumb these days is that a
simple QR Code (i.e., one representing a reasonably short URL) should be at least 3/4" square, which will meet the needs of most smartphone cameras and code reader software. If you don't want to use bit.ly or some other URL shortener, you can purchase a short, random domain name -- or you can work with a company (like my client,
Print 2D) that offers mobile subdomains and their corresponding printable, scan-able tags.
However, I realize this does not solve the dilemma for QR Codes containing contact data rather than mobile Web links. The more data you encode, the greater the code's pixel count. Shrinking these monsters down does increase the risk of reader failure -- but so does adding so much data that a smartphone camera can't cope.
I think we're facing two different adoption curves. For mobile Web access, QR Codes are at or near the tipping point in North America. For conveying other types of useful data -- like a vCard -- we're still at the mercy of handset manufacturers and their camera capabilities. We're also in the VERY early stages of code reader software. I'm still looking for the reader that will scan codes AND know what to do with them, regardless of what hardware I'm using.
