This depends on a lot of factors. However, to give you an idea; I know that a company called Xeamos has produced a SCR unit for a 1088 kW engine. The dimensions of this unit are 3,1 x 1 x 1 meter.
Yeah! The efficiency of the SCR unit depends on three important factors. Firstly, the temperature of the exhaust gasses. Secondly, the type and volume of the catalyst. And third, the control strategy, this determines the amount of urea that is mixed with the exhaust gasses.