The following is for a 6-1, but since a 24-2 is four times a 6-1, you can simply multiply the number and get an approximate size. I worked it to about 269 square inches of 4 row automotive type radiator for a 6-1 at full power and in equalibrium. Four times that is 1,076 square inches of 4 row radiator, that's about 33 inches square, 33" x 33". This assumes a fan of considerable vigor. "Truck" is a good suggestion, I think, for a 24-2. So is a very robust fan.
I did as follows: The specific fuel consumption for a 6-1 is given ordinarily at 255 g per kwh. Assuming this is true and that the given engine is at full power when it is making 4 kw at the flywheel, that's 1,020 grams per hour of fuel, releasing about 34.8 megajoules. (A liter of fuel is about 850 g, and fuel releases about 40.9 megajoules per liter). A kilowatt-hour is the equivalent to 3.6 megajoules, and 4 kwh is therefore equal to 14.4 megajoules, giving a (claimed) efficiency of about 41%. Not bad.
Anyway, 1 BTU is about 1,060 joules, and 34.8 megajoules is therefore about 36,888 BTU. That is about the maximum amount of heat that must be removed to stabilize a 6-1 running at 4 kw, if the factory fuel data is true. Obviously 1/4 of that, 1 kw, would require 9,222 BTU per hour of heat removal. This assumes that the engine is just as efficient at 1 kw as it is at 4 kw, that can't be true, but it ought to be fairly linear.
A surface radiator, a simple closed tank, to cool that would be (given Lionel Marks' "handbook"* 200 BTU per sq foot figure) require about 185 square feet of radiating surface, or more, depending primarily on air temperature.
For an automotive type fin-and-tube "radiator", assuming Lionel Marks' "Handbook" figure of 6-8 square inches of surface per BTU/MIN, and changing minutes to hours, that's 60 BTU per hour for 6-8 square inch of fin-and-tube surface, or, assuming 7, about 4,304 square inches of fin-and-tube to cool a 6-1 at full power. That's about 65.5 inches square, but as automotive fin-and-tubes have more than one row of "surface", or tubes, and we can assume 4 rows, that's something like 65.5/4, or about 16.4" x 16.4" of 4 row fin-and-tube a 269 square inch "radiator". A big heater core or a little radiator... 'course a fan is necessary to make that work and there must be some air flow rate. I would assume something like 44 feet per second, that's about 30 mph. That's a lot of fan... less fan means more fin-and-tube, maybe half the fan means twice the fin-and-tube, that remains to be found out.
An implication is that the factory cooling tank systems must rely on evaporation to a considerable extent for cooling, as the tank dimensions Lister gives do not provide enough radiant surface to cool an engine of this size and efficiency, not, at least, according to the published data.
*Lionel S. Marks, Mechanical Engineer's Handbook, 4th ed.