Using the open-cell photoacoustic technique, we have measured the room-temperature thermal diffusivities of the colossal magnetoresistive material La0.67Ca0.33MnO3, sintered between 1100degC and 1350degC, with average grain sizes 1, 3, 5, and 10 mum. We obtained the thermal diffusivities by analyzing the phase of photoacoustic signals in thermally thick samples using Calderon's method. We found that the insulator-metal transition temperature does not depend on the grain size (TIM ~ 272 K). However, the thermal diffusivity increases with grain size, with values between 0.431 and 0.969 mm2 s-1. Other related electrical and thermal properties, including the electrical conductivity, thermal conductivity, and phonon mean free path, are also dependent on the grain size. The electronic contribution to the thermal conductivity is 2%-3% of the total thermal conductivity for smaller grain sizes (1-5 mum) and increases to about 24% when the grain size is increased to 10 mum.