Canon 1Ds MIII has a 21.0MP Full frame (36 x 24 mm ) sized CMOS sensor . On the other hand, Canon 1D MII N has a 8.0MP APS-H (28.7 x 19.1 mm ) sized CMOS sensor .
Canon 1Ds MIII's sensor provides 13MP more than Canon 1D MII N'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 1Ds MIII and 1D MII N sensor size comparison.
As seen above, Canon 1Ds MIII has a 1.6x Larger sensor area than Canon 1D MII N. 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
Both Canon 1Ds MIII and Canon 1D MII N sensors have been tested by DxoMark. DxoMark 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. Of the two cameras that we are comparing, 1Ds MIII has scored 80, 14 points higher than 1D MII N.