It all depends on what kind of roads you are driving, are you driving in 4" => snow? The winter tires make the big difference. if you are driving in under 4" of snow then 18" and even 19" of snow will do the job.

Few members here have snow tires on 20" wheels and it does the job for them.
http://www.xoutpost.com/85592-post1.html
http://www.janixworld.com/x5/Wheels/

I often go snowboarding and leave the car at around 1600-1700m, I can't tell how much snow because it may start snowing while being there and nobody would clean those mountain roads before, maybe, the next day!.

17's
 

I have 17's with Nokian Hakka's. Can't beat em in the winter!

buy my 18's, 17's look like crap

I agree, the 17's are ugly. But performance-wise, they are supposed to be better in the snow.