Electrical resistivity measurements have been carried out on flash evaporated (Bi0.4Sb0.6)2Te3 thin films. From the temperature dependence of electrical-resistivity, the activation energy for conduction in the films was calculated and it was found to be thickness dependent. The reciprocal thickness dependence of electrical resistivity observed has been explained by the `effective mean free path model' of classical size effect. The energy band gap has been calculated from the optical absorbance data for different thickness films. The observed thickness dependence of optical energy band gap has been explained based on the quantum size effect.