Both Nikon D800 and Nikon D810 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 D810'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 D800 and D810 sensor size comparison.
Nikon D800 and Nikon D810 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.