Something, people often tend to forget (e.g. QR codes on black background).

QR Code FAQ

The size of QR Code depends on a module size and a symbol version. The module refers to the black and white dots that make up the QR Code. The symbol version determines the data capacity.

The area of a Version 1 QR Code is made up of 21 modules per side. Each higher version number is made up of 4 additional modules. The highest version number is Version 40.

For example, a Version 2 QR Code would be composed of a 25 by 25 module matrix. If the module size is 0.5 mm (19.7 mils) square, the size of QR Code will be 12.5 mm (0.5") square. (QR Code requires a four module wide quiet zone around the symbol.)

With white space this isn't a problem at all, see QR Code on the right.
Our experience is that it works almost always with only one module, but if you want to be on the safe side, it's good to keep this in mind.