radiator size for pc cooling

many stores give radiator dimensions for water cooling system. quick google tells me I need 120mm per either a component or 100w, ok.
But the radiator sizes given on websites are 120, 240, 360 etc instead they give stuff like 402 x 120 x 27 mm or 317 x 140 x 27 etc which number am I supposed to be looking at or am I misunderstanding something