I recommend Tirerack, as well.
You would be ok with a 17" square setup. Downsizing for winter improves snow traction and it's cheaper.
I have my Rial Salernos + Dunlop SP Winter Sport M3 (225/45R17) from them since 2007 and they've been pretty good to me. Used them on my previous '07 E92 328 Sport, planning to use them on my '10 E90 M-Sport.
Prices are decent and the quality is fine.