Leica SL2-S has a
24.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor and features Maestro III processor. On the other hand, Nikon D6 has a
21.0MP Full frame (35.9 x 23.9 mm ) sized CMOS sensor and features Expeed 6 processor.
Another difference between these two cameras is that Leica SL2-S'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 moiré occurring in certain scenes.
Below you can see the SL2-S and D6 sensor size comparison.
Leica SL2-S and Nikon D6 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.