In a continuous process connected to another continous process, a surge drum is often installed between them to allow variations in one process to be isoloated from the other process. For instance if PLANT1 output goes into PLANT2 with a surge drum, when PLANT2 slows down or speeds up it allows time for PLANT1 to smoothly change rates by allowing surge drum level to "surge".
Sizing involves knowing how much "surge time" is required and the "volume" of the surge.