Sony Alpha a6400 has a APS-C sensor with dimensions of 23.5 x 15.6 mm and sensor area of 366.60mm2.
It is 16% smaller than the average Mirrorless 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 Sony A6400 Review