Canon EOS-1D Mark II has a APS-H sensor with dimensions of 28.7 x 19.1 mm and sensor area of 548.17mm2.
Canon 1D MII's sensor is 20% larger than the average DSLR sensor in its class. In cases where all the other parameters are the same between two cameras, the one with the larger sensor collects more light per pixel and performs better (less noise) in low light / High ISO conditions . Larger sensors also give the photographer more control on the depth of field.
Read our detailed Canon 1D MII Review