As rallycinq said, the most important is the compound, not the width of the tyre. When the tyre digs through the snow, it still has to grip what's beneath it. On my last car I had 245/45 R19 winter tyres fitted, and they worked perfectly. I would suggest buying decent quality winter tyres, as in my experience you get what you pay for, and if you look after them, they'll last for several winters no problem. I originally bought a cheap set of tyres, that lasted literally 3 months, and were scary to drive in the snow. I then bought a set of Continentals, and they lasted for 4 winters, driving across Europe several times, and they were also fine for motorway driving. Try the Camskill website.