Downscaling of polycrystalline silicon (polysilicon) thin film transistors (TFTs) is currently pursued to improve device speed, and in the present work we analyze the effects of downscaling on the self-heating related instability. Experimental results show improved stability of scaled polysilicon TFTs operating at same power densities. To explain this effect we performed three-dimensional thermo-electric numerical simulations, indicating that channel temperature reduction occurs in the scaled devices due to enhanced lateral thermal dissipation. Therefore, the present results show that downscaling not only provides higher speed devices but also a route for reducing self-heating related instability in polysilicon TFTs.