You must do an energy balance between hot and cold fluids to determine the area requirement of the exchanger and the desired maximum temperature of the glycol.  First select the desired discharge temperature of the exhaust gas and the enthalpy change that results, including any condensation of moisture.  Select an operating temperature for the glycol which is satisfactory for heating in the building and calculate the flow rate based on the desired maximum glycol temperature and the enthalpy recovered from the generator exhaust in the heat exchanger.