Since this site is probably the only place autistic to care, I remember reading in one of the novels the dimensions of Mirrodin and the wiki supports it.
Mirrodin has a circumference of 1400 kms and a diameter of 450 km.
For reference, the moon has a circumference of 10, 921 km. The earth= 40, 075 km. I can't remember the actual matg, but Mirrodin is pathetically small. Even if you gave New Phyrexia every benefit of the doubt. A multi-plane invasion was the most retarded plan anyone could possibly conceive( authors included). Especially, considering, Dominaria is twice the size of earth. The oil was their greatest weapon, and with time could have easily infected the entire multiverse using sleeper agents and defending from planeswalker attack. Mark me mad at the internet, but using war as a set piece without any of the strategy is a pet peeve of mine. War is atrocious, but full of good story opportunities.