Both Canon T6i and Canon SL2 have APS-C sized 24.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the T6i and SL2 sensor size comparison.
Canon T6i and Canon SL2 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Canon T6i and Canon SL2 sensors have been tested by DxO and the results show that SL2 has a better overall score of 79, 8 points higher compared to T6i's score of 71.