An optical model for Raman response of thin metal films has been developed, taking into account attenuation effects and multi-reflections occurring at film edges. Film thickness and surface morphology of nanometer thin NbN films have been inferred in the framework of this model using micro-Raman measurements. Results have been compared with those obtained by means of spectroscopic ellipsometry. The absolute value of the thickness is determined with a precision better than 20% on nanometric scale allowing us to control surface morphology with high accuracy also on large areas.