The wheels on the M5 are forged alloy and are either 19" (1) or 20" (2). This dimension refers to the diameter of the wheel itself, not the wheel+tire diameter. On my M5 I have 19" wheels for the winter tires, and 20" for the summer tires.
It is important that the wheels be as lightweight as possible, because they act like a gyroscope and resist turning if too heavy. Also, they are part of the so-called "unsprung" mass of the car, that part of the car which is isolated from the heavier chassis. Lighter unsprung mass means that the wheels will more readily conform to the road surface, promoting better traction.
The summer tires are Michelin Pilot Super Sports. The front tires are 265/35R20 at the front, and 295/30R20 at the rear. The "295" part is the width of the tire in millimetres, or 11.6" wide. The "30" part is the ratio of tire width to tire sidewall height. So the sidewalls are 295 * 30% = 88.5mm = 3.5". Therefore the diameter of the tire and wheel together is 27". The summer tires are rated for very high speed, and the tire has been customized by Michelin specifically for the F10 M5. The tires would be extremely poor for winter driving, as the grippy rubber compound becomes hard like a hockey puck when too cold.
The winter tires are Pirelli Winter 240 Sottozer Series 2.
They are 255/40R19 all around. The narrower tire is better for traction in the snow. It concentrates the weight of the car onto a smaller patch to allow for better grip in the snow. However traction on dry pavement is less good due to the smaller contact patch with the road. The rubber, however, stays soft even in the cold. The tire is optimized for winter driving, high speeds, and great handling.
Unlike on most BMWs, the tires are not "run-flat" models. The very stiff sidewall on a run-flat compromises grip and handling, and so is not deemed suitable for an "M" car. However, neither is there a spare tire in the trunk due to space and weight considerations. Instead we get a "kit" that attempts to seal and re-inflate a punctured tire.