The number you enter as the Read Block Size defines the number of records fetched by the component during a single simulation step. Set the Write Block Size to define the number of records to be written.
Most data provider components possess a Read Block Size property. You can customize the Write Block Size for most of the data sink components. You can customize both reading and writing values for transformation components, such as the Staging component.
The Block Size property is evaluated during project simulation as well as project and job execution. A small number might be suitable for simulation purposes, but slows execution. In simulation, the Block Size is restricted to 32K.