I ran the Pirelli's in the OEM size (19" wheels 255/40f, 285/35r) on my E60 M5. I could not even back out of our driveway without spinning the tires in the snow. With 400+ lbs of weight in the trunk, the snow performance was tolerable. If you want to have reasonable traction in the snow, FOLLOW THE BMW RECOMENDATIONS! The recommendation was to run '4-square' 255/40-19's all around as the previous poster did. I didn't, because I really hated the way the Blizzaks felt in the dry, and then I switched to more performance oriented snow tires, and still didn't like the dry handling. I went with the OEM setup because I got much better handling in the dry. I would not do this if you want any kind of traction in the snow - the rear tires are just too wide.
Here's how 'wide tires' were explained to me in one of my driving schools at Mid-Ohio. If you look at the 'physics' of tires, all tires have a contact patch with the road. The contact patch is simply the cars weight divided by the air pressure in the tires. If you're running the same air pressure in wide and narrow tires, you have the same amount of rubber contacting the road. The difference is that a wide tire has a narrow and wide patch that is ideal for cornering. A narrow tire has a long, narrow patch that is less prone to 'hydroplaning', or riding up onto the snow/rain. This is the reason you want to use the narrowest possible tire in the rear when driving in snow. The larger, softer sidewall doesn't transmit the axle torque to the road as directly as a smaller, stiffer sidewall, and so makes the wheel less likely to spin.