Both Nikon Z6 III and Leica SL2-S have Full frame sized 24.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
One other difference between these two cameras that is worth mentioning is that Leica SL2-S's sensor doesn't have an 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 Z6 Mark III and SL2-S sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon Z6 III and Leica SL2-S Cameras
Nikon Z6 III and Leica SL2-S 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.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Nikon Z6 III
or the
Leica SL2-S ,
including hyperfocal distance.
