Best Practices for Sizes and Resolution
We have posted a couple of blog articles on that topic. The bottom line is this though --- as the quality of the camera inside of phones continues to increase, you will be able more easily scan smaller QR Codes.
But as of now, here are some things that we've found:
o This blog is about using QR Codes that point to a URL
o This blog is about QR Codes containing contact information
Please keep in mind that in addition to size, the readability can also depend on the material that the QR Code is printed on, the mobile device being used, as well as the application being used. As such, we highly recommend testing your QR Code first.
Also, our QReate & Track site has a lot of helpful information if you would like to check that out as well! http://qreateandtrack.com/