Akavir is smaller than Tamriel, but bigger than Old Ehlnofey. It's just an idea, albeit one that occasionally sends armies to invade the 'real' world of Tamriel.
As Don't Forget This pointed out, when Nirn was first created, it was mainly land with seas. All the current landmasses (Yokuda, Tamriel, Akavir, Atmora, maybe some other stuff that I forgot/haven't been discovered) were together in one Pangaea-like supercontinent. THAT is Ehlnofey. And your second sentence just doesn't make any sense. Akavir is just as real as Tamriel, certainly not "just an idea." And that has no relation to size. Unless you've invented a new definition for the word without telling any of us.