By mistake Nikon leaked these four images of their new Full Frame mirrorless. It shows the huge mount and the big grip of the new camera.
And this is a very rudimental and probably not 100% inaccurate comparison. But a reader sent me this based on the rumored Nikon 3mm larger mount diameter. Not that we learn much from this but the Nikon could (repeat could!) be slightly larger than the A7 series. Also it seems the Nikon has a LCD display on top.
UPDATE: Here are more full size images via MirrorlessRumors.com:
The new Nikon camera will be announced on August 23.