Thin film size effects cause strong anisotropy and reduction in electrical and thermophysical material properties. This paper studies these effects and the effect of the active region radius on the temperature rise in vertical-cavity surface-emitting laser diodes (SELDs). The temperature rise in the active region of an external-cavity SELD is determined experimentally by measuring the laser output wavelength as a function of the injection current and substrate temperature. Simulation results based on models for the current flow and heat generation agree well with experimental data and they demonstrate that the thin film size effects have a strong influence on the laser temperature rise. The effect of the active region radius is clearly demonstrated by the existence of an optimal value for minimal device temperature rise.