Canon 1D MII has a 8.0MP APS-H (28.7 x 19.1 mm ) sized CMOS sensor . On the other hand, Nikon Z6 has a 25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 6 processor.
Nikon Z6's sensor provides 17MP more than Canon 1D MII's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 1D MII and Z6 sensor size comparison.
As seen above, Nikon Z6 has a 1.6x Larger sensor area than Canon 1D MII. 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.
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 1D MII and Nikon Z6 sensors have been tested by DxO and the results show that Z6 has a better overall score of 95, 29 points higher compared to 1D MII's score of 66.