Depends on the wheel specs as to what tyres to put on, but 195/55/16 would probably be about right.
Assuming they are for your 318d then 16's should fit fine. If you can though, try fitting one to the front before purchase just to be sure they will clear.
Nokian make a VERY good winter tyre :
http://lovetyres.com/tyre/Nokian-WR-D3/195-55-16
Or you can stick to the bigger players as mentioned above.