LOL, look what I've found!
Somebody had the same idea and made a picture of it.
However, this pic is made up to suggest that you can see our past, for example dinosaurs. This would require 1 mio years of travelling, setting up the mirror, and another 1 mio years after the mirror has started reflecting, for the light to come back to earth. So, it doesn't work like that but we could see in 2 mio years from now, well, what has happened today at earliest. uhm
PS: The mirror wouldn't necessarily have to be really big, 1 millimeter size would already do it.