Electron transport through square arrays of antidots in a magnetic field is numerically studied for various antidot diameters. Using the linear response theory, we find two features: (1) At the magnetic field of the main peak in the resistivity, there is an optimum antidot diameter at which the conductivity is maximized. (2) In the region of weak magnetic field, negative Hall conductivity is observed. The range of the magnetic field in which the conductivity is negative becomes large when the antidot diameter is increased. Further the origins of the above features are clarified in connection with the classical trajectories of the electron.