I read in several M3 forums that the standard 18" 245 winter tires are really rubbish.
Especially on snow with Michelin Alpin tires.
About the size: On my dad’s 997 turbo the Pirelli winter tires (235/295) are really awesome! I don’t think that smaller tires are better on snow anymore.
But my biggest issue is the fact, that from behind, it would look like you've mounted wheelbarrow tyres. So tiny inside those massive wheel wells
I guess I will go for the standard BMW E92 M3 260-styling rimes with 245/265 tires.