Delivr Corporation offers a QR code generator for web and print use. Here are their guidelines for sizing:
QR Code Printing Guidelines & Best Practices
32 × 32 mm or 1.25 × 1.25 inches, excluding quiet zone (the white space included around the QR Code in the EPS) is the minimum size that guarantees that ALL camera phones on the market can properly read the QR Code.
26 × 26 mm or roughly 1 square inch, excluding quiet zone covers 90% of the phones on the market. – OUR CURRENT RECOMMENDATION
The latest camera models, which have improved macro capabilities, can deal with QR Codes that are less than 10 mm (0.4 in) wide and high.
For example, an iPhone 4 will be able to scan this size, but an iPhone 2/3.x with 2.0 megapixel camera and no auto-focus may not.
For good reader accuracy good contrast between the background and the bar color itself is very important. The bar code should have a dark color on a light background. You cannot go wrong by treating the QR Code as line art, using black on white.
QR Codes printed at 37.5cm (14.764 inches) x 37.5cm (14.764 inches) using several popular QR Code Readers across iPhone, Android, and BlackBerry had a successful scanning distance varying between 5′ – 19′. Most all QR Code readers worked at 7′-10′.”