As long as you are running a full water jacket and do not have a leaking head gasket, any engine size will not overheat in parade duty with a "good" radiator, fan, and thermostat.
You have to remember that the cooling system only has to extract the heat being rejected to the water jacket to maintain heat balance. Parade duty would only require a few horsepower, so therefore the radiator does not have to reject much heat. By using a thermostat, you raise the temperature of the coolant and that allows the radiator to more easily reject that heat.
Anything up to (but not including) 5" bore runs a full water jacket in the H260 block.