Both Canon T6i and Nikon D3500 have APS-C 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 Nikon D3500'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 T6i and D3500 sensor size comparison.
Sensor Size and Resolution Comparison image of Canon T6i and Nikon D3500 Cameras
As seen above, Nikon D3500 has a 1.1x Larger sensor area than Canon T6i. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in 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
Canon T6i
or the
Nikon D3500 ,
including hyperfocal distance.
