Hi, I have an 08 C350 4Matic with the 18" AMG wheel option.

The front wheels are 225/40 -18 - RIM 8" width
The rears are 255/40 - 18 - RIM 8.5" width

I think I was told that winters should not be used in staggered formation and tire rack.com shows 225/40-18 as the "option 18" size (but the 4matic 350 isn't available in the US)

So...end of the day...

what size tires do I need for these wheels??

ideally, you should get another set of wheels for the winter that are not staggered. For winter tires, you want the narrower size, so 225/40-18 all around. Since the rim width is pretty close, you should be ok.

ideally you should get another set of wheels that aren't 18" for winter.....does leave a lot of tread for traction

Ya I know. I just really like the 18" wheels and the winters are manageable here. I ran all seasons on my IS250 with AWD with no problems whatsoever.

i'm not quite understanding your question.
you already know the tire size and yet, you are asking for tire size?

doesn't really matter to use staggered for winter or not.
it's just fact that wider tires have less traction on snow.

Just get the 225s. As others have suggested, an additional set of snow rims and tires would be easier to manage. You don't have to mount and unmount your tires. Mind you, low profile 18" snow tires aren't cheap.

Lucky you, there was more snowfall in Richmond Hill / Aurora last winter.

You should talk to tirerack to see what they recommend if you intend to use the OE wheels.

I think 225/40/18 would be too narrow for 8.5" wheel which equals to about 215mm.
The tyre sidewall will sit really flush with the wheel.