For my winter setup, I went with Blizzak WS90 on the oem wheels and the stock size (235/40R18). I had no problems and I do commute a lot from near Montreal to Trois-Rivière.
I think the smallest you could go is 17 inch wheels from what I found because of the brakes. With the smaller wheels you'd cut through the snow more but I didn't felt like I needed it for my usage.