Both Nikon D800E and Nikon D810A have Full frame sized 36.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another difference between these two cameras is that Nikon D800E's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the D800E and D810A sensor size comparison.
Nikon D800E and Nikon D810A sensors have exactly the same size so none of them has any advantage over other in this area.