There seems to be some sort of obsession with fitting the largest diameter wheels to cars that are available. I sometimes feel that the style of a wheel dictates how good the wheel looks rather than the size. This is the reason I stuck to Alfa 17" alloys (5 hole style) rather than any 18s that I was considering, even the Zender Sienas.
I'm sure everyone has their own opinion about what constitutes 'good looking' on a car, but the Alfa 17s in my opinion suit the car more than most of the 18s. I've heard from someone who's fitted 18" Sienas that there is a noticeable difference in ride quality, with some rub on the back when carrying rear passengers.
Also, most 18s are 8" wide and so stick out some way from the body of the car.
Lastly, since Alfa don't make any 18" alloys, I would stick with Alfa 17s - I'm not too keen on aftermarket wheels on Alfa Romeos, especially since Alfa's own alloys are some of the best looking/most stylish around.
I hope this has confused the whole issue somewhat !