There are multiple guys at tire rack. I am usually the one that people talk to because I work extensively with the M3's due to my involvement with the forum.
I defintely have recommended the 255/40R18 for the winter. Typically that would be the rear size in a staggered setup.
To credit the forum, click through one of the banners in my signature and the top of the page, or call me direct at 877.522.8473 ext 660.