There is no written document stating what actually determines a combine's class size. It is "assumed" that a combination of specifications for various systems and areas that make up a combine, most commonly engine horsepower and grain tank capacity. Example, the lexion 480 or 485R, rated at 400hp with 280bu grain tank = class 8 status, as well as the new New Holland CR 970, at 370hp and 330bu grain tank. No real logic there when no range of size is published to determine such a class.