This would depend on the capacity (CFM) of your air compressors, the required pressure and the total length of piping in the system. By knowing those three factors, standard tables can be used to determine the size of pipe required to minimize pressure drop. It is also important to know the number of valves and elbows in the system, as each one counts as an equivalent length of straight pipe. Size piping for the maximum CFM required and include the current system capacity and possible future expansion in your piping plan.