Nikon D850 has a Full frame sensor with dimensions of 35.9 x 23.9 mm and sensor area of 858.01mm2.
Nikon D850's sensor is 87% 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 Nikon D850 Review