I noticed on my M6 that the wipers are two different sizes (bigger on left than on the right)... Is this right or should they be the same size? While we are at it, what size should they be?

While I'm not sure what size your MS6 should have, it is fairly common that the left and right wipers are different sizes. I don't remember ever replacing wiper blades and them being the same size.

More often then not they are different sizes. It all depends on the car.

I once got a 1" larger size for my sisters Mustang on the driver side and it minimized that dead-zone at the bottom and ended up wiping past the top of the windshield. Almost a clear view all around. You just want to make sure they don't hit if you go up a size.

Lots of cars are like that. On my SRT-4 I think it was like 21" left and 16" right. The Miata is the same size on both, but that's not always the case.