The function of an expansion tank is to absorb the expansion and contraction of the heating fluid caused by temperature change in an RFH system.
The proper size for an expansion tank depends on the volume of heating fluid in the system and the temperature differential. RFH systems, by nature, have relatively low temperature differentials. The most common pipe diameter, 1/2"(16 mm outside diameter), contains about one gallon of fluid per hundred feet. A #15 expansion tank will service a 2,000' system, a #30 will service up to a 5,000' system, and a #60 will service a 10,000' system. Boiler volumes and operating temperatures can significantly change expansion tank requirements. Manufacturers provide selection data for proper sizing.