Both Canon 5DS and Canon 5DS R have Full frame sized 51.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another difference between these two cameras is that Canon 5DS R'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 5DS and 5DS R sensor size comparison.
Canon 5DS and Canon 5DS R have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.