It's hard to find any concrete details on the resolutions used by GE, and it varies massively by area. I recall their recent investment in one of the (GeoEye?) satellites had a theoretical resolution of 40cm, but is restricted by US regulations to provide a maximum (edit: minimum) pixel size of either 50 or 60cm.
Assuming 1m x 1m pixels, a standard[1] QR code could be fitted inside a 25x25m square. You'd probably want to increase pixel size, and maybe use a smaller version (along with a url shortener, if necessary) to increase the chances of a successful decode.
Aircraft orthoimaging can work at much higher resolutions. An article from 2005[2] shows pictures capable of distinguishing individual faces. I can only assume things have since improved.
[1] of which there are many: https://secure.wikimedia.org/wikipedia/en/wiki/QR_code
[2] http://www.gearthblog.com/blog/archives/2005/09/ultimate_goo...