Estimates from pictures range from 475 to 560mm2You have a source on the size?
Point is, it is a massive change from regular GCN CUs. Even if potential for high performance is there, it won't all be there at launch.
I predict Vega will not reach stock 1080Ti performance apart from couple edge cases. And it will overclock worse.
Being 1 year late to the party already, they need to be very smart with pricing.