The wider the wheels and the tires the more understeer they will induce. Some people simply want the widest wheels & tires possible so they would go with something like 20x9 in the front and 20x11 rear. If you're just looking for a staggered setup in 20's I would go with 20x8.5 in the front and 20x10 rear. Another point is that you won't want to run bigger wheels on small widths because it will affect the car's handling as well as ride (e.g. 20x8 all around).
Here's a set of 20" Vorsteiner 3 piece forged wheels on an E92 M3 for your viewing pleasure.