For all plumbing, larger pipe will usually help increase the efficiency of the pump but only up to a point. Using 2.5" pipe for all pool plumbing is a good idea including using separate runs for the suction side so that the main drain and each skimmer has it's own line from pool to the equipment pad. You can get away with smaller pipe but the up grade usually doesn't cost that much more and will probably save you energy costs in the long run.