I know it's a little early but I looked into what size winter tires might fit the BRZ.
According to Tire Rack; We show you can do a 16" with a 205/55/16 or 215/50/16 tire size.
I am a fan of Nokian snows and will put them on even though my car will see only light use in over the winter months.
I love rear drive winter driving but the car is too low to clear even plowed new england roads - and too low to see and be seen over and around snow piles and drifts at corners.