Following on from Paddy's comment above, personally I think that 18" rims suit the E39 perfectly. My first E39 ran 17" style 42s which I left on because somehow the design appeared to make them look bigger. Maybe the sport suspension helped there as it sat lower of course.
My second and current E39 started with 17" style 66s, a good looking wheel but unfortunately, as Dan says, not made in 18" flavour. The car is now fitted with 18" style 65s and to me looks just right. If you look in my Mohne Dam gallery here you will see the car with both the 66s and 65s fitted as a visual comparison.
Keep an eye out in the wheels and tyres section here. Something might appear, or you could put a shout out in the wanted section.