The switching characteristics of a vanadium dioxide (VO₂) thin-film device, in which the current flowing through the device can be switched through the photothermal effect using focused laser pulses, were investigated according to the dimensions of the sapphire substrate on which the VO₂ thin film was deposited through simulation using COMSOL Multiphysics. The physical properties of the VO₂ device, modeled for the simulation, were determined according to the structural and electrical properties and photothermally controlled current-switching characteristics of fabricated VO₂ devices. For a variety of substrate dimensions of the modeled VO₂ device, we explored transient variations in the temperature of some specific regions and the device current switched by laser irradiation. The investigation results revealed that the stability of the bidirectional current-switching operation triggered on and off by laser illumination tends to increase as the area of the substrate increases with its thickness fixed. However, above a certain substrate area, the rate of improvement in the switching stability decreases rapidly and approaches zero.